MODEM  KOAD  CONSTRUCTION 


A  PRACTICAL  TREATISE  ON  THE  ENGINEERING  PROBLEMS  OF 
ROAD  BUILDING,  WITH  CAREFULLY  COMPILED  SPECI- 
FICATIONS FOR  MODERN  HIGHWAYS,  AND 
CITY  STREETS  AND  BOULEVARDS 


AUSTIN  T.  BYRNE 

CIVIL  ENGINEER 

AUTHOR   OF 

'HIGHWAY  CONSTRUCTION,"  "MATERIALS  AND  WORKMANSHIP' 


ILLUSTRATED 


AMERICAN  TECHNICAL  SOCIETY 

CHICAGO 

1917 


COPYRIGHT,  1917,  BY 

AMERICAN  TECHNICAL  SOCIETY 

COPYRIGHTED   IN   GREAT  BRITAIN 
ALL  RIGHTS  RESERVED 


INTRODUCTION 

THE  science  of  good  road  building  is  an  old  one  as  evidenced 
by  the  many  highways  in  Europe  which  have  withstood  the 
wear  of  travel  for  centuries.  Most  of  these  famous  roads  were 
cut  from  solid  rock  or  built  of  crushed  stone  of  such  a  character 
as  to  be  unaffected  by  weather  conditions.  Modern  road  building, 
however,  has  been  largely  influenced  within  the  past  fifteen  years 
by  the  enormous  increase  in  the  amount  of  travel  due  to  the  auto- 
mobile. This  has  not  only  been  the  means  of  developing  new  road 
surface  to  meet  the  more  severe  requirements  of  this  type  of  vehicle 
but  it  has  developed  a  country-wide  interest  in  good  roads,  thus 
making  it  possible  for  the  enthusiastic  travelers  to  take  long  tours 
without  meeting  the  formerly  ever-present  bugaboo  of  bad  roads, 
besides  making  the  ordinary  town-to-town  travel  more  satisfactory. 

<I  It  is  with  the  idea  of  giving  a  clear  conception  of  the  engineering 
problems  involved  in  road  building,  that  is,  laying  out  of  the  road 
by  the  best  and  easiest  route,  the  questions  of  grade,  contour, 
and  drainage,  and  the  construction  of  culverts  and  bridges,  that 
this  treatise  has  been  written.  The  author  has  had  long  experience 
in  the  field  of  highway  construction  and  has  treated  the  different 
types  of  roads  in  a  very  complete  and  practical  manner.  Natural 
soil,  gravel,  broken  stone,  bituminous  macadam  and  concrete 
roads  are  all  carefully  treated,  not  only  as  to  material,  but  as  to  the 
best  methods  of  laying  them.  The  city  pavements  are  also  given 
due  consideration,  accompanied  by  typical  specifications  for  the 
new  surfaces  developed  for  boulevards. 

<I  Altogether,  the  article  covers  the  entire  field  of  road  building, 
both  city  and  country,  and  should  appeal  either  to  the  highway 
engineer  or  to  the  untrained  reader  who  has  merely  a  passing 
interest  in  the  subject. 


365550 


CONTENTS 

COUNTRY  ROADS  AND  BOULEVARDS 

PAGE 

Resistance  to  movement  of  vehicles 1 

Resistance  to  traction 1 

Tractive  power  and  gradients ." 7 

Axle  friction 11 

Resistance  of  air 12 

Location  of  roads 12 

Reconnoissance 13 

Preliminary  survey 15 

Topography 15 

Map 20 

Memoir 22 

Bridge  sites 22 

Final  selection  of  route 22 

Preliminary  road  construction  methods. . 35 

Width  and  transverse  contour 35 

Drainage 38 

Types  of  drainage 38 

Nature  of  soils 39 

Location  of  drains 39 

Proper  fall  for  drains 40 

Materials  used  for  drains 40 

Sizes  of  drams 42 

Silt  basins 42 

Protection  of  drain  ends  from  weather 42 

Drain  outlets. 43 

Side  ditches 43 

Treatment  of  springs  found  in  cuttings 44 

Drainage  for  hillside  roads 45 

Inner  and  outer  road  gutters 45 

Culverts 46 

Earthenware  pipe  culverts 49 

Iron  pipe  culverts.  . . 51 

Box  culverts 52 

Arch  culverts 53 

Short  span  bridges  used  as  culverts 53 

Earthwork 55 

Balancing  cuts  and  fills 55 

Side  slopes 55 

Shrinkage  of  earthwork 57 

Prosecution  of  earthwork 58 

Methods  of  forming  embankments 58 


CONTENTS 

Preliminary  road  construction  methods  (Continued)  PAGE 

Tools  for  construction  work 60 

Natural-soil  roads 74 

Earth  roads 74 

Sand  roads 77 

Sand-clay  roads 77 

Application  of  oil  to  sand  and  gravel  soils 78 

Roads  with  special  coverings 79 

Foundations 79 

Materials 79 

Thickness 79 

Types  of  foundation  to  be  used '. 81 

Wearing  surfaces 82 

Maintenance  and  improvement  of  roads 109 

Repair  and  maintenance  of  broken-stone  roads 109 

Systems  of  maintenance 1 10 

Improvement  of  existing  roads 110 

Traffic  census Ill 

CITY  STREETS  AND  HIGHWAYS 
Foundations ' 121 

Stone-block  pavements ,   123 

Materials 124 

Cobblestone  pavement 125 

Belgian-block  pavement 125 

Granite-block  pavement 126 

Blocks .' ,....• 127 

Manner  of  laying  blocks 127 

Foundation 129 

Cushion  coat 129 

Laying  blocks 130 

Ramming 130 

Fillings  for  joints 130 

Stone  pavement  on  steep  grades 132 

Brick  pavements 133 

Qualifications  of  brick 133 

Tests  for  paving  brick 136 

Brick-pavement  qualifications 137 

Foundation • 137 

Sand  cushion 137 

Manner  of  laying 138 

Joint  fillings 139 

Tools  used  by  hand  in  the  construction  of  block  pavements 145 

Concrete-mixing  machine 145 

Gravel  heaters.  .  146 


CONTENTS 

PAGE 

Wood-block  pavements 147 

Creosoting • 147 

Laying  the  blocks 149 

Qualifications  of  wood  pavements 152 

Asphalt  pavements 153 

Sheet-asphalt  pavement 153 

Laying  the  pavement.  . 155 

Foundation 157 

Qualifications  of  asphalt  pavements 157 

Failure  of  asphalt  pavement 159 

Rock  asphalt  pavement 161 

Asphalt  blocks ! 161 

Tools  employed  in  construction  of  asphalt  pavements. 162 

Miscellaneous  pavements '164 

Burnt  clay 164 

Straw 164 

Oyster-shell 164 

Chert 164 

Slag 164 

Clinker 165 

Petrolithic 165 

Kleinpflaster 165 

Iron 165 

Trackways 165 

National  pavement 166 

Filbered  asphalt  pavement 166 

Miscellaneous  street  work 166 

Curbstones  and  gutters 170 

Curbstones 170 

Combination  curb  and  gutter 171 

Street  cleaning 172 

Cleaning  methods 172 

Removal  of  snow 175 

Street  sprinkling 176 

Selecting  the  pavement 176 

Qualifications 176 

Interests  affected 177 

Problem  involved  in  selection 177 

Economic  benefit 181 

Relative  economies.  \ 181 

Gross  cost  of  pavements 184 

Comparative  rank  of  pavements 185 

Specifications 185 

Contracts . .  ...  187 


HIGHWAY  CONSTRUCTION 

PART  I 


COUNTRY  ROADS  AND  BOULEVARDS 

RESISTANCE  TO  MOVEMENT  OF  VEHICLES 

The  object  of  a  road  is  to  provide  a  way  for  the  transportation 
of  persons  and  goods  from  one  place  to  another  with  the  least 
expenditure  of  power  and  expense.  The  facility  with  which  this 
traffic  or  transportation  may  be  conducted  over  any  given  road 
depends  upon  the  resistance  offered  to  the  movement  of  vehicles. 
This  resistance  is  composed  of:  (1)  resistance  offered  by  the  road- 
way, which  consists  of  (a)  "friction"  between  the  surface  of  the 
road  and  the  wheel  tires,  (b)  resistance  offered  to  the  rolling  of 
the  wheels  occasioned  by  the  want  of  uniformity  in  the  road  surface 
or  lack  of  strength  to  resist  the  penetrating  efforts  of  loaded  wheels, 
(c)  resistance  due  to  gravity  called  "grade  resistance";  (2)  resistance 
offered  by  vehicles,  termed  "axle  friction";  and  (3)  resistance  of 
the  air.  The  magnitude  of  each  of  the  components  has  a  wide 
range,  varying  with  the  kind  and  condition  of  the  road  and  its 
surface,  the  form  and  condition  of  the  vehicle,  the  load,  and  the 
speed. 

Resistance  to  Traction.  The  combination  of  road  resistances 
is  designated  by  the  general  term  "resistance  to  traction",  the 
magnitude  of  which  is  measured  by  the  number  of  pounds  of  effort 
per  ton  of  the  load  required  to  overcome  it;  this  is  ascertained  by 
a  form  of  spring-balance  variously  called  "dynograph",  "tracto- 
graph",  etc.,  one  end  of  which  is  attached  to  the  vehicles  and  the 
other  end  to  the  draft  animals. 

The  road  which  offers  the  least  resistance  to  traffic  should 
combine  a  surface  on  which  the  friction  of  the  wheels  is  reduced 
to  the  least  possible  amount,  while  possessing  sufficient  roughness 
to  afford  good  foothold  for  the  draft  animals  and  good  adhesion 
for  motor  vehicles;  and  should  be  so  located  as  to  give  the  most 
direct  route  with  the  least  gradients. 


HIGHWAY  CONSTRUCTION 


TABLE  I 
Resistance  to  Traction  on  Different  Road  Surfaces 


ROAD  SURFACE 

V 

TRACTION  RESISTANCE 

Pounds  per  Ton 

In  Terms  of  Load 

Earth  road  —  ordinary  condition 
Gravel 
Sand 
Macadam 
Plank  road 
Steel  wheelway 

50  to  200 
50  to  100 
100  to  200 
30  to  100 
30  to    50 
15  to    40 

4T0      tO   A 
4*0-     tO    2*0 

^0    tO  T"O 
17     tO.  ^5 

iV    to^ 
ik  tO  rif 

Friction.  The  resistance  of  friction  arises  from  the  rubbing 
of  the  wheel  tires  against  the  surface  of  the  road;  its  amount  can 
be  determined  only  by  experiment  for  each  kind  of  road  surface. 
From  many  experiments  the  following  deductions  are  drawn: 

(1)  The  resistance  to  traction  is  directly  proportional  to  the  pressure. 

(2)  On  solid  unyielding  surfaces,  the  resistance  is  independent  of  the 
width  of  the  tire;  but  on  compressible  surfaces  it  decreases  as  the  width  of  the 
tire  increases.     There  is  no  material  advantage  gained,  however,  in  making 
a  tire  more  than  4  inches  wide,  for  the  reason  that  it  is  impossible  to  distribute 
the  load  evenly  over  the  road  owing  to  the  irregularities  and  curvatures  of  its 
surface. 

(3)  On  uniformly  smooth  surfaces,  the  resistance  is  independent  of  the 
speed. 

(4)  On  rough  irregular  surfaces,  which  give  rise  to  constant  concussion, 
the  resistance  increases  with  the  speed. 

Table  I  shows  the  relative  resistance  to  traction  of  various 
surfaces.  These  coefficients  refer  to  the  power  required  to  keep  the 
load  in  motion.  It  requires  from  two  to  six  or  eight  times  as  much 
force  to  start  a  load  as  it  does  to  keep  it  in  motion  at  two  or  three 
miles  per  hour.  The  extra  force  required  to  start  a  load  is  due  in 
part  to  the  fact  that  during  the  stop  the  wheel  may  settle  into  the 
road  surface;  in  part  to  the  fact  that  the  axle  friction  at  starting 
is  greater  than  after  motion  has  begun;  and  in  part  to  the  fact 
that  energy  is  consumed  in  accelerating  the  load. 

Resistance  to  Rolling.  Resistance  to  rolling  is  caused  partly 
by  the  wheel  penetrating  or  sinking  below  the  surface  of  the  road, 
forming  a  depression  or  rut,  as  shown  in  Fig.  1,  thus  compelling 
the  wheel  to  be  continually  rolling  up  a  short  incline.  The  measure 
of  this  resistance  is  the  horizontal  force  necessary  at  the  axle  to 


HIGHWAY  CONSTRUCTION 


roll  it  up  the  incline;  and  is  equal  to  the  product  of  the  load  multi- 
plied by  one-third  of  the  semi-chord  of  the  submerged  arc  of  the 
wheel. 

Resistance  to  rolling  is  also  caused  by  the  wheel  striking  or 
colliding  with  loose  or  projecting  stones,  which  suddenly  checks 


Road  Surface 


fJl  Rest  3 


Natural  Soil/  ^Rood  Covering 

Fig.  1.     Exaggerated  Section  of  Road  under  Pressure  of  Loaded  Vehicle 

the  motive  power;  the  momentum  thus  destroyed  varies  with  the 
height  of  the  stone  or  obstacle  and  is  often  considerable. 

In  both  cases  the  power  required  to  overcome  the  resistance 
is  affected  largely  by  the  diameter  of  the  wheel,  as  the  larger  the 
wheel  the  less  force  is  required  to  lift  it  over  the  obstruction  or 
to  roll  it  up  the  inclination  due 
to  the  indentation  of  the  sur- 
face. 

Illustrative  Example.  The 
power  required  to  draw  a  wheel 
over  ^a  stone  or  any  obstacle, 
such  as  S  in  Fig.  2,  may  be  thus 
calculated : 

Let  P  represent  the  power 
sought,  or  that  which  would 
just  balance  the  weight  on  the 
point  of  the  stone,  and  the  slightest  increase  of  which  would  draw 
it  over.  This  power  acts  in  the  direction  CP  with  the  leverage  of 
BC  or  DE.  The  force  of  gravity  W  resists  in  the  direction 
CB  with  the  leverage  BD.  The  equation  of  equilibrium  will  be 
PXCB  =  WXBD,  whence 


R  5 

Fig.  2.     Diagram  for  Calculating  Power 

Required  to  Draw  Wheel  over 

Resisting  Object 


BD= 

CB  CA 


AB 


4  HIGHWAY  CONSTRUCTION 

Let  the  radius  of  the  wheel  equal  CD  =  26  inches,  and  the  height 
of  the  obstacle  equal  AB  =  4:  inches.  Let  the  weight  11'  =  500 
pounds,  of  which  200  pounds  may  be  the  weight  of  the  wheel  and 
300  pounds  the  load  on  the  axle.  The  formula  then  becomes 

p     gnnV676-484     rm  13.85 
P  =  500 — —     -  =500 ——— =314.7  Ib. 
2o  — 4  22 

The  pressure  at  the  point  D  is  compounded  of  the  weight  and  the 
power,  and  equals 

CD  26 

H/li=500xl=5911b- 

Therefore  this  pressure  acts  with  this  great  effect  to  destroy  the  road 
in  its  collision  with  the  stone;  in  addition  there  is  to  be  considered 
the  effect  of  the  blow  given  by  the  wheel  in  descending  from  it. 
For  minute  accuracy  the  non-horizontal  direction  of%  the  draft  and 
the  thickness  of  the  axle  should  be  taken  into  account.  The  power 
required  is  lessened  by  proper  springs  to  vehicles,  by  enlarged  wheels, 
and  by  making  the  line  of  draft  ascending. 

Illustrative  Example.  The  mechanical  advantage  of  the  wheel 
in  surmounting  an  obstacle  may  be  computed  from  the  principle 

of  the  lever.  Let  the  wheel, 
Fig.  3,  touch  the  horizontal  line 
of  traction  in  the  point  A  and 
meet  a  protuberance  ED.  Sup- 
pose the  line  of  draft  CP  to  be 
parallel  to  AB.  Join  CD  and 
draw  the  perpendiculars  DE  and 
DF.  We  may  suppose  the  power 

Fig.  3.     Force  Diagram  for  Wheel  Drawn  to  be  applied  at  E  and  the  Weight 

over  Obstacle  ,,  .  •      ,1  (1 

at  F,  and  the  action  is  then  the 

same  as  the  bent  lever  EDF  turning  round  the  fulcrum  at  D.  Hence 
P  :  W  ::  FD  :  DE.  But  FD  :  DE  ::  tan  FCD  :  1 ;  and  tan  FCD  = 
tan  2  DA  B;  therefore  P  =  Wtsm2DAB.  Now  it  is  obvious  that  the 
angle  DAB  increases  as  the  radius  of  the  circle  diminishes;  there- 
fore, the  weight  W  being  constant,  the  power  required  to  overcome 
an  obstacle  of  given  height  is  diminished  when  the  diameter  is 
increased.  Large  wheels  are,  therefore,  the  best  adapted  for  sur- 
mounting inequalities  of  the  road. 


HIGHWAY  CONSTRUCTION 


TABLE  II 
Resistance  Due  to  Gravity  on  Different  Inclinations 


Grade  1  inch 

20 

30 

40 

50 

60 

'  70 

80 

90 

100 

200 

300 

400 

Rise  in  feet  per  mile 

264 

176 

132 

105 

88 

75 

66 

58 

52 

26 

17 

13 

Resistance  in  pounds 

per  ton 

100 

66$ 

50 

40 

33f 

28| 

25 

221 

20 

10 

6f 

5 

There  are,  however,  circumstances  which  provide  limits  to 
the  height  of  the  wheels  of  vehicles.  If  the  radius  AC  exceeds 
the  height  of  that  part  of  the  horse  to  which  the  traces  are  attached, 
the  line  of  traction  CP  will  be  inclined  to  the  horse,  and  part  of  the 
power  will  be  exerted  in  pressing  the  wheel  against  the  ground. 
The  best  average  size  of  wheels  is  considered  to  be  about  6  feet  in 
•  diameter.  Wheels  of  large  diameter  do  less  damage  to  a  road  than 
small  ones,  and  cause  less  draft  for  the  horses.  With  the  same  load, 
a  two-wheeled  cart  does  far  more  damage  than  one  with  four  wheels, 
and  this  because  of  their  sudden  and  irregular  twisting  motion  in 
the  trackway. 

Grade  Resistance.  Grade  resistance  is  due  to  the  action  of 
gravity,  and  is  the  same  on  good  and  bad  roads.  On  level  roads 
its  effect  is  immaterial,  as  it  acts  in  a  direction  perpendicular  to 
the  plane  of  the  horizon  and  neither  accelerates  nor  retards  motion. 
On  inclined  roads  it  offers  considerable  resistance,  proportional  to 
the  steepness  of  the  incline.  The  resistance  due  to  gravity  on  any 
incline  in  pounds  per  ton  is  equal  to 

2000 
rate  of  grade 

Table  II  shows  the  resistance  due  to  gravity  on  different  grades. 
The  additional  resistance  caused  by  inclines  may  be  investigated 
in  the  following  illustrated  example. 

Illustrative  Example.  Suppose  the  whole  weight  to  be  borne 
on  one  pair  of  wheels,  and  that  the  tractive  force  is  applied  in  a 
direction  parallel  to  the  surface  of  the  road. 

Let  AB,  Fig.  4,  represent  a  portion  of  the  inclined  road,  C 
being  a  vehicle  just  sustained  in  its  position  by  a  force  acting  in  the 
direction  CD.  It  is  evident  that  the  vehicle  is  kept  in  its  position 
by  three  forces :  namely,  by  its  own  weight  W  acting  in  the  vertical 
direction  CF;  by  the  force  F  applied  in  the  direction  CD  parallel 


6 


HIGHWAY  CONSTRUCTION 


Fig.  4.     Forces  Acting  on  Vehicle  When 
Drawn  up  Inclined.  Road 


to  the  surface  of  the  road;  and  by  the  pressure  P  which  the  vehicle 
exerts  against  the  surface  of  the  road  acting  in  the  direction  CE 
perpendicular  to  the  same.  *  To  determine  the  relative  magnitude 
of  these  three  forces,  draw  the  horizontal  line  AG  and  the  vertical 
line  BG;  then,  since  the  two  lines  CF  and  BG  are  parallel  and  are 
both  cut  by  the  line  AB,  they  must  make  the  two  angles  CFE  and 
ABG  equal;  also  the  two  angles  CEF  and  AGE  are  equal;  there- 
fore, the  remaining  angles  FCE  and  BAG  are.  equal,  and  the  two 
triangles  CFE  and  ABG  are  similar.  And  as  the  three  sides  of 

the  former  are  proportional  to 
the  three  forces  by  which  the 
vehicle  is  sustained,  so  also 
are  the  three  sides  of  the  lat- 
ter; namely,  the  length  of  the 
road  AB  is  proportional  to 
W,  or  the  weight  of  the 
vehicle;  the  vertical  rise  BG 
is  proportional  to  F,  or  the 
force  required  to  sustain  the 
vehicle  on  the  incline;  and  the  horizontal  distance  AG  in  which 
the  rise  occurs  is  proportional  to  P,  or  the  force  with  which  the 
vehicle  presses  upon  the  surface  of  the  road.  Therefore, 

W : AB  : : F : GB 
and 

W : AB : : P : AG 

If  to  AG  such  a  value  be  assigned  that  the  vertical  rise  of  the 
road  is  exactly  one  foot,  then 

W  W 


and 


in  which  A  is  the  angle  BAG. 

To  find  the  force  requisite  to  sustain  a  vehicle  upon  an  inclined  road 
(the  effects  of  friction  being  neglected),  divide  the  weight  of  the  vehicle 
and  its  load  by  the  inclined  length  of  the  road,  the  vertical  rise  of 
which  is  one  foot,  and  the  quotient  is  the  force  required. 


HIGHWAY  CONSTRUCTION 


TABLE  III 
Tractive  Power  of  Horses  at  Different  Velocities 


MILES  PER  HOUR 

TRACTIVE  FORCE 
db.) 

MILES  PER  HOUR 

TRACTIVE  FORCE 
(lb.) 

I 

333.33 

21 

111.11 

250 

2£ 

100 

11 

200 

2| 

90.91 

H 

166.66 

3 

83.33 

if 

142.86 

3£ 

71.43 

2 

125 

4 

62.50 

To  find  the  pressure  of  a  vehicle  against  the  surface  of  an  inclined 
road,  multiply  the  weight  of  the  loaded  vehicle  by  the  horizontal 
length  of  the  road,  and  divide  the  product  by  the  inclined  length 
of  the  same;  the  quotient  is  the  pressure  required.  The  force 
with  which  a  vehicle  presses  upon  an  inclined  road  is  always  less 
than  its  actual  weight;  the  difference  is  so  small  that,  unless  the 
inclination  is  very  steep,  it  may  be  taken  equal  to  the  weight  of  the 
loaded  vehicle. 

To  find  the  resistance  to  traction  in  passing  up  or  down  an  incline, 
ascertain  the  resistance  on  a  level  road  having  the  same  surface  as 
the  incline,  to  which  add,  if  the  vehicle  ascends,  or  subtract,  if 
it  descends,  the  force  requisite  to  sustain  it  on  the  incline;  the  sum 
or  difference,  as  the  case  may  be,  will  express  the  resistance. 

Tractive  Power  and  Gradients.  Although  transportation  by 
mechanically  propelled  vehicles  will  continue  to  increase,  it  is  not 
probable  that  for  many  years  it  will  become  more  important  than 
traffic  drawn  by  animals;  and  as  mechanically  propelled  vehicles 
can  ascend  any  grade  feasible  for  animals,  it  is  only  necessary  to 
discuss  the  effect  of  grades  on  horse-drawn  traffic. 

Tractive  •  Power  of  Horses.  The  necessity  for  easy  grades  is 
dependent  upon  the  power  of  the  horse  to  overcome  the  resistance 
to  motion,  which  is  composed  of  four  forces,  viz,  friction,  collision, 
gravity,  and  resistance  of  air.  All  estimates  on  the  tractive  power 
of  horses  must  to  a  certain  extent  be  vague,  owing  to  the  different 
strengths  and  speeds  of  animals  of  the  same  kind,  as  well  as  to  the 
extent  of  their  training  to  any  particular  kind  of  work. 

The  draft  or  pull  which  a  good  average  horse,  weighing  1,200 
pounds,  can  exert  on  a  level,  smooth  road  at  a  speed  of  2  J  miles  per 


HIGHWAY  CONSTRUCTION 


TABLE  IV 
Variation  of  Tractive  Power  with  Time 


HOURS  PER  DAY 

TRACTIVE  FORCE 
db.) 

HOURS  PER  DAY 

TRACTIVE  FORCE 

db.) 

10 

100 

7 

146$ 

9 

111* 

6 

166| 

8 

125 

5 

200 

NOTE:  The  tractive  power  of  teams  may  be  found  by  multiplying  the 
above  values  by  the  following  constants: 

1  horse  =1 

2  horses     0.95X2  =  1.90 

3  horses     0.85X3=2.55 

4  horses     0.80X4  =  3.20 

hour  is  100  pounds;  which  is  equivalent  to  22,000  foot-pounds  per 
minute,  or  13,200,000  foot-pounds  per  day  of  10  hours.  The  tractive 
power  diminishes  as  the  speed  increases  and,  perhaps,  within 
certain  limits,  say  from  f  mile  to  4  miles  per  hour,  nearly  in  inverse 
proportion  to  it.  Thus  the  average  tractive  force  of  a  horse,  on  a 
level,  and  actually  pulling  for  10  hours,  may  be  assumed  approx- 
imately as  shown  in  Table  III. 

The  work  done  by  a  horse  is  greatest  when  the  velocity  with 
which  he  moves  is  one-eighth  of  the  greatest  velocity  with  which 
he  can  move  when  unloaded;  and  the  force  thus  exerted  is  0.45 
of  the  utmost  force  that  he  can  exert  at  a  dead  pull.  The  tractive 
power  of  a  horse  may  be  increased  in  about  the  same  proportion 
as  the  time  is  diminished,  so  that  when  working  from  5  to  10  hours 
on  a  level,  it  will  be  about  as  shown  in  Table  IV. 

Loss  of  Tractive  Power  on  Inclines.  In  ascending  inclines  a 
horse's  power  diminishes  rapidly;  a  large  portion  of  his  strength  is 
expended  in  overcoming  the  resistance  of  gravity  due  to  his  own 
weight  and  that  of  the  load.  Table  V  shows  that  as  the  steepness 
of  the  grade  increases,  the  efficiency  of  both  the  horse  and  the  road 
surface  diminishes;  that  the  more  of  the  horse's  energy  which  is 
expended  in  overcoming  gravity,  the  less  remains  to  overcome  the 
surface  resistance. 

Table  VI  shows  the  gross  load  which  an  average  horse,  weighing 
1,200  pounds,  can  draw  on  different  kinds  of  road  surfaces,  on  a 
level  and  on  grades  rising  5  and  10  feet  per  100  feet. 


HIGHWAY  CONSTRUCTION 


TABLE  V 

Effects  of  Grades  upon  the  Load  a  Horse  Can  Draw  on  Different 

Pavements 


GRADE 

EARTH 

BROKEN  STONE 

STONE  BLOCKS 

ASPHALT 

Level 

1.00 

1.00 

1.00 

1.00 

1  100 

.80 

.66 

.72 

.41 

2  100 

.66 

.50 

.55 

.25 

3  100 

.55 

.40 

.44 

.18 

4  100 

.47 

.33 

.36 

.13 

5  100 

.41 

.29 

.30 

.10 

10  100 

.26 

.16 

.14 

.04 

15  100 

.10 

.05 

.07 

20  100 

.04 

.03 

The  decrease  in  the  load  which  a  horse  can  draw  upon  an  incline 
is  not  due  alone  to  gravity;  it  varies  with  the  amount  of  foothold 
afforded  by  the  road  surface.  The  tangent  of  the  angle  of  inclination 
should  not  be  greater  than  the  coefficient  of  tractional  resistance. 
Therefore,  it  is  evident  that  the  smoother  the  road  surface,  the  easier 
should  be  the  grade;  the  smoother  the  surface  the  less  the  foothold, 
and  consequently  the  less  the  possible  load. 

The  loss  of  tractive  power  on  inclines  is  greater  than  any  inves- 
tigation will  show;  for,  besides  the  increase  of  draft  caused  by 
gravity,  the  power  of  the  horse  is  much  diminished  by  fatigue  upon 
a  long  ascent,  and  even  in  greater  ratio  than  man,  owing  to  its 
anatomical  formation  and  great  weight.  Though  a  horse  on  a 
level  is  as  strong  as  five  men,  on  a  grade  of  15  per  cent,  it  is  less 
strong  than  three;  for  three  men  carrying  each  100  pounds  will 
ascend  such  a  grade  faster  and  with  less  fatigue  than  a  horse  with 
300  pounds. 

A  horse  can  exert  for  a  short  time  twice  the  average  tractive 
pull  which  he  can  exert  continuously  throughout  the  day's  work; 
hence,  so  long  as  the  resistance  on  the  incline  is  not  more  than  double 
the  resistance  on  the  level,  the  horse  will  be  able  to  take  up  the  full 
load  which  he  is  capable  of  drawing. 

Steep  grades  are  thus  seen  to  be  objectionable,  and  particularly 
so  when  a  single  one  occurs  on  an  otherwise  comparatively  level 
road,  in  which  case  the  load  carried  over  the  less  inclined  portions 
must  be  reduced  to  what  can  be  hauled  up  the  steeper  portion. 

The  bad  effects  of  steep  grades  are  especially  felt  in  winter, 


10 


HIGHWAY  CONSTRUCTION 


TABLE  VI 
Gross  Loads  for  Horse  on  Different  Pavements  on  Different  Grades 


DESCRIPTION  OF  SURFACE 

LEVEL 

GRADE 
(5  per  cent) 

GRADE 
(10  per  cent) 

Asphalt 

13,216 

Broken  stone  (best  condition) 

6,700 

1^840 

1^060 

Broken  stone  (slightly  muddy) 

4,700 

1,500 

1,000 

Broken  stone  (ruts  and  mud) 

3,000 

1,390 

890 

Broken  stone  (very  bad  condition) 

1,840 

1,040 

740 

Earth  (best  condition) 

3,600 

1,500 

930 

Earth  (average  condition) 

1,400 

900 

660 

Earth  (moist  but  not  muddy) 

1,100 

780 

600 

Stone-block  pavement  (dry  and  clean) 

8,300 

1,920 

1,090 

Stone-block  pavement  (muddy) 

6,250 

1,800 

1,040 

Sand  (wet) 

1,500 

675 

390 

Sand  (dry) 

1,087 

445 

217 

when  ice  covers  the  roads,  for  the  slippery  condition  of  the  surface 
causes  danger  in  descending,  as  well  as  increased  labor  in  ascending; 
during  heavy  rains  the  water  also  runs  down  the  road  and  gulleys 
it  out,  destroying  its  surface,  thus  causing  a  constant  expense  for 
repairs.  The  inclined  portions  are  subject  to  greater  wear  from 
horses  ascending,  thus  requiring  thicker  covering  than  the  more 
level  portions,  and  hence  increasing  the  cost  of  construction. 

It  will  rarely  be  possible,  except  in  a  flat  or  comparatively  level 
country,  to  combine  easy  grades  with  the  best  and  most  direct 
route.  These  two  requirements  will  often  conflict.  In  such  a  case, 
increase  the  length  of  the  road.  The  proportion  of  this  increase 
will  depend  upon  the  friction  of  the  covering  which  is  adopted. 
But  no  general  rule  can  be  given  to  meet  all  cases  as  respects  the 
length  which  may  thus  be  added,  for  the  comparative  time  occupied 
in  making  the  journey  forms  an  important  element  in  any  case 
which  arises  for  settlement.  Disregarding  time,  the  horizontal 
length  of  a  road  may  be  increased  to  avoid  a  5  per  cent  grade,  seventy 
times  the  height. 

Table  VII  shows,  for  most  practical  purposes,  the  force  required 
to  draw  loaded  vehicles  over  inclined  roads.  In  the  fifth  column 
the  length  given  is  the  length  which  would  require  the  same  motive 
power  to  be  expended  in  drawing  the  load  over  it,  as  would  be 
necessary  to  draw  over  a  mile  of  the  inclined  road.  The  loads 
given  in  the  sixth  column  are  the  maximum  loads  which  average 


HIGHWAY  CONSTRUCTION 


11 


TABLE  VII 
Data  for  Loaded  Vehicles  over  Inclined  Roads 


Rate  of  Grade 
(ft.  per  100 

ft.) 

Pressure  on 
Plane 
(Ib.  per  ton) 

Tendency 
down  Plane 
(Ib.  per  ton) 

Power 
Required  to 
Haul  1  Ton 
up  Plane 
(Ib.) 

Equivalent 
Length  of 
Level  Road 
(mi.) 

Maximum 
Load  Horse 
Can  Haul 

(Ib.) 

0.00 

2240 

.00 

45.00 

1.000 

6270 

0.25 

2240 

5.60 

50.60 

1.121 

5376 

0.50 

2240 

11.20. 

56.20 

1.242 

4973 

0.75 

2240 

16.80 

61.80 

1.373 

4490 

1.00 

2240 

22.40 

67.40 

1.500 

4145 

1.25 

*2240 

28.00 

73.00 

1.622 

3830 

1.50 

2240 

33.60 

78.60 

1.746 

3584 

1.75 

2240 

39.20 

84.20 

1.871 

3290 

2.00 

2240 

45.00 

90.00 

2.000 

3114 

2.25 

2240 

50.40 

95.40 

2.120 

2935 

2  .  50. 

2240 

56.00 

101.00 

2.244 

2725 

2.75 

2240 

61.33 

106.33 

2.363 

2620 

3.00 

2239 

67.20 

112.20 

2.484 

2486 

4.00 

2238 

89.20 

134.20 

2.982 

2083 

5.00 

2237 

112.00 

157  .  00 

3.444 

1800 

6.00 

2233 

134.40 

179  .  40 

3.986 

1568 

7.00 

2232 

156.80 

201  .  80 

4.844 

1367 

8.00 

2232 

179.20 

224  .  20 

4.982 

1235 

9.00 

2231 

201.60 

246.60 

4.840 

1125 

10.00 

2229 

224.00 

269.00 

5.977 

1030 

*  Near  enough  for  practice;  actually  2239.888 
Pressure  on  plane  ==  weight Xnat  cos  of  angle  of  plane 

horses  weighing  1,200  pounds  can  draw  over  such  inclines,  the  friction 
of  the  surface  being  taken  at  -§V  of  the  load  drawn. 

Axle  Friction.  The  resistance  of  the  hub  to  turning  on  the 
axle  is  the  same  as  that  of  a  journal  revolving  in  its  bearing,  and  has 
nothing  to  do  with  the  condition  of  the  road  surface.  The  coefficient 
of  journal  friction  varies  with  the  material  of  the  journal  and  its 
bearing,  and  with  the  lubricant.  It  is  nearly  independent  of  the 
velocity,  and  seems  to  vary  about  inversely  as  the  square  root  of  the 
pressure.  For  light  carriages  when  loaded,  the  coefficient  of  friction 
is  about  0.020  of  the  weight  on  the  axle;  for  the  ordinary  thimble- 
skein  wagon  when  loaded,  it  is  about  0.012.  These  coefficients  are 
for  good  lubrication;  if  the  lubrication  is  deficient,  the  axle  friction 
is  2  to  6  times  as  much  as  above. 

The  tractive  power  required  to  overcome  the  above  axle  friction 
for  carriages  of  the  usual  proportions  is  about  3  to  3f  pounds  per 
ton  of  the  weight  on  the  axle;  and  for  truck  wagons,  which  have 
medium  sized  wheels  and  axles,  it  is  about  3J  to  4J  pounds  per  ton. 


12 


HIGHWAY  CONSTRUCTION 


TABLE  VIII 
Wind  Pressure  for  Various  Vehicles 


DESCRIPTION 

VELOCITY  OF  WIND 
(mi.  per  hour) 

WIND  PRESSURE 
(Ib.  per  sq.  ft.) 

Pleasant  breeze 

15 

1.107 

Brisk  gale 

20 
25 

1.968 
3.075 

High  wind 

30 
35 

4.428 
6.027 

Very  high  wind 

40 
45 

7.782 
9.963 

Storm 

50 

12.300 

Effect  of  Springs  on  Vehicles.  Experiments  have  shown  that 
springs  mounted  in  vehicles  materially  decrease  the  resistance  to 
traction  and  diminish  the  effects  caused  in  the  vertical  plane  by 
irregularities  of  the  surface ;  but  they  do  not  diminish  the  horizontal 
component  which  is  the  one  that  causes  the  greatest  wear  of  the 
road,  especially  at  speeds  beyond  a  walking  pace.  The  vehicles 
with  springs  were  found  not  to  cause  more  wear  with  the  horses 
going  at  a  trot  than  vehicles  without  springs  when  the  horses  were 
walking,  all  other  conditions  being  similar.  Vehicles  with  springs 
improperly  fixed  cause  considerable  concussion  which,  in  turn, 
destroys  the  road  covering. 

Resistance  of  Air.  The  resistance  arising  from  the  force  of 
the  wind  will  vary  with  the  velocity  of  the  wind,  with  the  velocity 
of  the  vehicle,  with  the  area  of  the  surface  acted  upon,  and  also 
with  the  angle  of  incidence  of  direction  of  the  wind  with  the  plane 
of  the  surface.  Table  VIII  gives  the  force  per  square  foot  for 
various  velocities. 


LOCATION  OF  ROADS 

The  considerations  governing  the  location  of  roads  are 
dependent  upon  the  commercial  condition  of  the  country  to  be  trav- 
ersed. In  old  and  long-inhabited  sections,  the  controlling  element 
will  be  the  character  of  the  traffic  to  be  accommodated.  In  such 
a  section,  the  route  is  generally  predetermined  and,  therefore,  there 
is  less  liberty  of  choice  and  selection  than  in  a  new  and  sparsely 
settled  district,  where  the  object  is  to  establish  the  easiest,  shortest, 


HIGHWAY  CONSTRUCTION  13 

and  most  economical  line  of  intercommunication  according  to 
the  physical  character  of  the  ground. 

Whichever  of  these  two  cases  may  have  to  be  dealt  with,  the 
same  principle  governs  the  engineer,  namely,  so  to  lay  out  the  road 
as  to  effect  the  conveyance  of  the  traffic  with  the  least  expenditure 
of  motive  power  consistent  with  economy  of  construction  and  main- 
tenance. 

Economy  of  motive  power  is  promoted  by  easy  grades,  by  the 
avoidance  of  all  unnecessary  ascents  and  descents,  and  by  a  direct 
line;  but  directness  must  be  sacrificed  to  secure  easy  grades  and  to 
avoid  expensive  construction. 

RECONNOISSANCE 

Object  of  Reconnoissance.  The  selection  of  the  best  route 
demands  much  care  and  consideration  on  the  part  of  the  engineer. 
To  obtain  the  requisite  data  upon  which  to  form  his  judgment, 
he  must  make  a  personal  reconnoissance  of  the  district.  This 
requires  that  the  proposed  route  be  either  ridden  or  walked  over 
and  a  careful  examination  made  of  the  principal  physical  contours 
and  natural  features  of  the  district.  The  amount  of  care  demanded 
and  the  difficulties  attending  the  operations  will  altogether  depend 
upon  the  character  of  the  country.  The  immediate  object  of  the 
reconnoissance  is  to  select  one  or  more  trial  lines,  from  which  the 
final  route  may  be  ultimately  determined.  When  there  are  no 
maps  of  the  section  traversed,  or  when  those  which  can  be  procured 
are  indefinite  or  inaccurate,  the  work  of  reconnoitering  will  be  much 
increased. 

Points  to  Consider.  In  making  a  reconnoissance  there  are  several 
points  which,  if  carefully  attended  to,  will  very  considerably  lessen 
the  labor  and  time  otherwise  required.  Lines  which  would  run  along 
the  immediate  bank  of  a  large  stream  must  of  necessity  intersect 
all  the  tributaries  confluent  on  that  bank,  thereby  demanding  a 
corresponding  number  of  bridges.  Those,  again,  which  are  situated 
along  the  slopes  of  hills  are  more  liable  in  rainy  weather  to  suffer 
from  the  washing  away  of  the  earthwork  and  the  sliding  of  the 
embankments;  the  others  which  are  placed  in  valleys  or  on  elevated 
plateaux,  when  the  line  crosses  the  ridges  dividing  the  principal 
watercourses,  will  have  steep  ascents  and  descents. 


14  HIGHWAY  CONSTRUCTION 

In  making  an  examination  of  a  tract  of  country,  the  first  point 
to  attract  notice  is  the  unevenness  or  undulation  of  its  surface, 
which  appears  to  be  entirely  without  system,  order,  or  arrangement; 
but  upon  closer  examination  it  will  be  perceived  that  one  general 
principle  of  configuration  obtains  even  in  the  most  irregular  countries. 
The  country  is  intersected  in  various  directions  by  main  watercourses 
or  rivers,  which  increase  in  size  as  they  approach  the  point  of  their 
discharge.  Towards  these  main  rivers  lesser  rivers  approach  on 
both  sides,  running  right  and  left  through  the  country,  and  into 
these,  again,  enter  still  smaller  streams  and  brooks.  The  streams 
thus  divide  the  hills  into  branches  or  spurs  having  approximately 
the  same  direction  as  themselves,  and  the  ground  falls  in  every 
direction  from  the  main  chain  of  hills  towards  the  watercourses, 
forming  ridges  more  or  less  elevated. 

The  main  ridge  is  cut  down  at  the  heads  of  the  streams  into 
depressions  called  gaps  or  passes;  the  more  elevated  points  are 
called  peaks.  The  water  which  has  fallen  upon  these  peaks  is  the 
origin  of  the  streams  which  have  hollowed  out  the  valleys.  Further- 
more, the  ground  falls  in  every  direction  towards  the  natural  water- 
courses, forming  ridges  more  or  less  elevated  running  between  them 
and  separating  from  each  other  the  districts  drained  by  the  streams. 

The  natural  watercourses  mark  not  only  the  lowest  lines,  but 
the  lines  of  the  greatest  longitudinal  slope  in  the  valleys  through 
which  they  flow.  The  direction  and  position  of  the  principal  streams 
give  also  the  direction  and  approximate  position  of  the  high  ground 
or  ridges  which  lie  between  them.  The  positions  of  the  tributaries 
to  the  larger  stream  generally  indicate  the  points  of  greatest  depres- 
sion in  the  summits  of  the  ridges  and,  therefore,  the  points  at  which 
lateral  communication  across  the  high  ground  separating  con- 
tiguous valleys  can  be  most  readily  made. 

Instruments  Used.  The  instruments  employed  in  reconnoiter- 
ing  are:  the  compass,  which  is  used  to  ascertain  direction;  the 
aneroid  barometer,  to  fix  the  approximate  elevation  of  summits, 
etc.;  and  the  hand  level,  to  ascertain  the  elevation  of  neighboring 
points.  If  a  vehicle  can  be1  used,  an  odometer  may  be  added,  but 
distances  can  usually  be  guessed  or  ascertained  by  time  estimates 
closely  enough  for  preliminary  purposes.  The  best  maps  obtainable 
and  traveling  companions  who  possess  a  local  knowledge  of  the 


HIGHWAY  CONSTRUCTION  15 

country,  together  with  the  above  outfit,  are  all  that  will  be  necessary 
for  the  first  inspection. 

PRELIMINARY  SURVEY 

The  routes  selected  through  the  reconnoissance  are  examined 
in  detail  by  a  survey  called  a  "preliminary  survey"  from  the  results 
of  which  the  exact  location  can  be  determined. 

Features  to  Be  Considered.  In  making  the  preliminary  survey, 
the  topographical  features  are  noted  to  the  right  and  left  of  the 
transit  line  for  a  convenient  distance.  The  data  required  for  drawing 
the  topography  are  obtained  by  levels  taken  with  a  leveling  instru- 
ment or  with  a  transit  provided  with  stadia  wires,  on  lines  per- 
pendicular to  the  transit  line  of  the  survey.  The  location  of  build- 
ings, fences,  streams,  roads,  railroads,  and  other  objects,  is  determined 
by  measurements  made  with  a  tape  on  lines  perpendicular  to  the 
survey  line;  or,  when  the  distance  to  the  object  required  is  con- 
siderable, the  location  is  found  by  angles  measured  from  two  stations 
on  the  transit  line  and  the  distance  is  measured  by  stadia.  The 
following  information  is  also  noted:  the  importance,  magnitude, 
and  direction  of  the  streams  crossed;  the  character  of  the  material 
to  be  excavated  or  available  for  embankments;  the  position  of 
quarries;  the  mode  of  access  thereto,  and  the  kind  of  stone;  the  posi- 
tion of  unloading  points  on  railroads;  and  any  other  information 
that  might  affect  a  selection. 

Topography.  Levels.  Levels  should  be  taken  along  the  course 
of  each  line,  usually  at  every  100  feet,  or  at  closer  intervals, 
depending  upon  the  nature  of  the  country.  In  taking  the  levels,  the 
heights  of  all  existing  roads,  railroads,  rivers,  or  canals  should  be 
noted.  "Bench  marks"  should  be  established  at  least  every  half 
mile,  that  is,  marks  made  on  any  fixed  object,  such  as  a  gate  post, 
side  of  a  house,  or,  in  the  absence  of  these,  a  cut  made  on  a  large 
tree.  The  height  and  exact  description  of  each  bench  mark  should 
be  recorded  in  the  level  book. 

Cross  Levels.  Wherever  considered  necessary,  levels  at  right 
angles  to  the  center  line  should  be  taken.  These  will  be  found 
useful  in  showing  what  effect  a  deviation  to  the  right  or  left  of  the 
surveyed  line  would  have.  Cross  levels  should  be  taken  at  the 
intersection  of  all  roads  and  railroads  to  show  to  what  extent,  if 


16 


HIGHWAY  CONSTRUCTION 


Fig.  5.     Contour  Map  Used  in  Road  Surveys 


HIGHWAY  CONSTRUCTION  17 

% 

any,  these  levels  will  have  to  be  altered  to  suit  the  levels  of  the 
proposed  road. 

Contours.  The  levels  of  the  transit  and  cross  lines  are  worked 
into  a  map  that  shows  the  irregularities  of  the  ground  with  reference 
to  its  elevations  and  depressions.  Various  methods  are  employed 
for  delineating  these  upon  paper.  For  the  purpose  of  the  engineer 
the  method  of  contours,  Fig.  5,  is  the  most  serviceable,  since  by 
it  the  true  shape  of  the  hills  and  valleys  can  be  shown. 

Contours  are  lines  drawn  through  the  points  of  equal  altitude; 
that  is,  every  point  of  the  ground  over  which  a  contour  line  passes 
is  at  a  certain  height  above  a  known  fixed  plane  called  the  "datum". 


Fig.  6.     Diagram  Showing  Method  of  Approximating  Elevation  of  Successive 
Contours  of  Inclined  Road 

Mean  sea  level  is  the  datum  plane  universally  employed;  when 
this  cannot  be  conveniently  used,  an  arbitrary  plane  is  adopted 
which  is  below  the  lowest  point  in  the  territory  under  survey. 

The  difference  of  elevation  between  adjacent  contour  lines 
is  called  the  "contour  interval";  this  may  be  one,  five,  ten,  or  more, 
feet.  Whatever  the  difference  adopted,  it  must  be  constant  for 
all  contours  on  the  same  map.  Contours  are  designated  by  their 
height,  expressed  in  feet,  above  the  datum  plane.  The  elevation 
of  each  contour  is  shown  in  figures  at  points  close  enough  together 
to  allow  the  eye  to  run  from  one  to  the  other  with  ease.  It  is  best 
to  break  the  contour  and  write  the  numbers  between  the  ends.  If 


18 


HIGHWAY  CONSTRUCTION 


Method  of  Showing  Contours  of  Banks 
.  of  Streams 


written  alongside,   the  numbers  should  be  placed  on  the  higher 

side  of  the  contour. 

The  theory  of  contours  is  given  in  order  that  no  error  will  be 

made  by  supposing  the 
slope  of  the  ground  from  a 
point  in  one  contour  to  a 
point  in  the  next,  to  be  a 
straight  line.  The  less  the 
contour  interval,  the  less 
error  will  be  made.  If  in 
Fig.  6  the  curved  line  AB 
represents  the  actual  surface 
of  the  ground,  and  points 


1,3,5,  the  elevation  of  suc- 
cessive contours,  the  broken 
line  1,3,5  will  represent  the 
assumed  ground  surface,  and  its  departure  from  the  line  AB  is  the 
error  introduced.  If  now  the  points  2,  4,  6  are  also  determined, 
or  the  contour  intervals  be  reduced  one-half,  the  assumed  slope 

is  1,  2,  S,  4,  5,  6,  which 
differs  less  from  the  line 
AB  than  the  line  1,  3,  5, 
and  hence  introduces  less 
error.  With  points  deter- 
mined at  very  short  inter- 
vals, the  error  is  practically 
eliminated. 

A  knowledge  of  the 
shape  of  the  ground  is 
obtained  from  the  distance 
of  the  contours  from  one 
another.  The  steeper  the 
slopes,  the  closer  will  the 
contours  be.  If  in  a  hill 
the  upper  contours,  near  the 
summit,  are  closer  together  than  those  near  the  bottom,  the  inter- 
vening ground  is  concave;  if  the  lower  contours  are  closer  than  the 
higher  ones,  the  intervening  ground  is  convex. 


Fig.  8. 


Method  of  Showing  Contour  of  Small 
Stream  or  Dry  River  Bed 


HIGHWAY  CONSTRUCTION 


19 


Every  contour  must  close  upon  itself  in  a  loop  or  else  must 
extend  unbroken  from  one  point  on  the  margin  of  the  map  to  some 
other  point  on  the  margin.  An  exception  is  made  in  the  case  of 
large  streams,  the  contour  on  each  bank  being  carried  up-stream 
until  it  cuts  the  water  surface,  when  it  is  dropped,  as  shown  in  Fig.  7. 
In  a  small  stream  or  dry  bed,  the  contour  crosses  at  the  point  where 
the  elevation  of  the  bed  is  that  of  the  contour,  as  shown  in  Fig.  8. 


too 


Fig.  9.     Typical  Profile  as  Obtained  from  Contour  Map 

Profile.  A  profile  is  a  longitudinal  section  of  the  route.  The 
profile  in  any  given  direction  is  easily  made  from  the  contour  map 
in  the  manner  shown  in  Fig.  9.  Assuming  that  a  profile  is  required 
along  the  line  AB,  the  contours  show  that  the  ground  rises  from 
A  to  By  and  also  that  a  small  isolated  elevation  occurs  at  C.  The 
short  distance  between  the  contours  near  B  indicates  that  the 
rise  is  steep.  To  obtain  the  profile,  draw  parallel  lines  at  a  distance 
apart  equal  to  the  vertical  interval  between  the  contours  on  any 
convenient  exaggerated  scale.  Number  these  lines  to  correspond 


20 


HIGHWAY  CONSTRUCTION 


pooy 


—OO'SS- 


-OI'S9- 


—09'frS- 


•OS'tff- 


poojj 


00 '09— 


with  the  numbers  on  the  contours. 
From  each  point  on  the  line  AB, 
where  it  intersects  a  contour,  draw 
vertical  lines  to  intersect  the  corre- 
sponding horizontal  line.  Connect 
the  several  points  thus  found,  remem- 
bering the  distinction  between  convex 
and  concave  surfaces.  The  profile 
thus  obtained  gives  the  relative  heights 
of  different  points  in  the  line  AB, 
but  it  does  not  give  the  true  gradient. 
The  true  gradient  cannot  be  repre- 
sented accurately  unless  the  vertical 
intervals  are  drawn  on  the  same  scale 
as  the  horizontal  scale  of  the  map. 
If  this  is  done,  the  elevations  will 
generally  be  so  minute  that  the 
profile  will  not  give  a  sufficiently 
striking  representation  of  the  surface 
features.  It  is,  therefore,  necessary 
to  exaggerate  the  vertical  scale  in  a 
certain  fixed  proportion.  A  conven- 
ient scale  is  400  feet  horizontal  and 
40  feet  vertical.  A  typical  prelimi- 
nary profile,  with  all  the  information 
which  it  is  supposed  to  give,  is  shown 
in  Fig.  10. 

Map.  The  map,  Fig.  11,  should 
show  the  lengths  and  direction  of 
the  different  portions  of  the  line, 
the  topography,  rivers,  water-courses, 
roads,  railroads,  and  other  matters 
of  interest,  such  as  town  and  county 
lines,  dividing  lines  between  property, 
timbered  and  cultivated  lands,  etc. 
Any  convenient  scale  may  be  adopted; 
400  feet  to  an  inch  will  be  found  the 
most  useful. 


HIGHWAY  CONSTRUCTION 


21 


00=l  Inch. 
Fig.  11.     Typical  Map  Showing  Layout  in  the  Region  of  the  Proposed  Road 


22  HIGHWAY  CONSTRUCTION 

Memoir.  The  descriptive  memoir  should  give  with  minute- 
ness all  information,  such  as  the  nature  of  the  soil,  character  of  the 
several  excavations  whether  earth  or  rock,  and  such  particular 
features  as  cannot  be  clearly  shown  upon  the  map  or  profile.  Special 
information  should  be  given  regarding  the  rivers  crossed,  as  to  their 
width,  depth  at  highest  known  flood,  velocity  of  current,  character 
of  banks  and  bottom,  and  their  angle  of  skew  with  the  line  of  road. 

Bridge  Sites.  The  question  of  choosing  the  site  of  bridges  is 
an  important  one.  If  the  selection  is  not  restricted  to  a  particular 
point,  the  river  should  be  examined  for  a  considerable  distance 
above  and  below  what  would  be  the  most  convenient  point  for 
crossing;  and  if  a  better  site  is  found,  the  line  of  the  road  must  be 
made  subordinate  to  it.  If  several  practicable  crossings  exist,  they 
must  be  carefully  compared  in  order  to  select  the  one  most 
advantageous.  The  following  are  controlling  conditions:  (1)  Good 
character  of  river  bed,  affording  a  firm  foundation.  If  rock  is 
present  near  the  surface  of  the  river  bed,  the  foundation  wdll  be 
easy  of  execution,  and  stability  and  economy  will  be  insured.  (2) 
Stability  of  river  banks,  thus  securing  a  permanent  concentration 
of  the  waters  in  the  same  bed.  (3)  Axis  of  bridge  at  right  angles 
to  direction  of  current.  (4)  Bends  in  rivers,  not  being  suitable 
localities,  to  be  avoided  if  possible.  A  straight  reach  above  the 
bridge  should  be  secured  if  possible. 

FINAL  SELECTION  OF  ROUTE 

Elements  Entering  into  Choice.  In  making  the  final  selection, 
the  following  principles  should  be  observed  as  far  as  practicable: 

(1)  To  follow  that  route  which  affords  the  easiest  grades. 
The  easiest  grade  for  a  given  road  will  depend  on  the  kind  of  covering 
adopted  for  its  surface. 

(2)  To  connect  the  places  by  the  shortest  and  most  direct 
route  commensurate  with  easy  grades. 

(3)  To  avoid  all  unnecessary  ascents  and  descents.     When  a 
road  is  encumbered  with  useless  ascents,  the  wasteful  expenditure 
of  power  is  considerable. 

(4)  To  give  the  center  line  such  a  position,  with  reference  to 
the  natural  surface  of  the  ground,  that  the  cost  of  construction 
shall  be  reduced  to  the  smallest  possible  amount. 


HIGHWAY  CONSTRUCTION  23 

(5)  To  cross  all  obstacles,  where  structures  are  necessary,  as 
nearly  as  possible  at  right  angles.     The  cost  of  skew  structures 
increases  nearly  as  the  square  of  the  secant  of  the  obliquity. 

(6)  To  cross  ridges  through  the  lowest  pass  which  occurs. 

(7)  To  cross  either  under  or  over  railroads;  for  grade  crossings 
mean  danger  to  every  user  of  the  highway. 

Treatment  of  Typical  Cases 

Connecting  Two  Towns   in  Same  or  Adjacent  Valleys.    In 

laying  out  the  line  of  a  road,  there  are  three  cases  which  may  have 
to  be  treated,  and  each  of  these  is  exemplified  in  the  contour  map, 
Fig.  5.  First,  the  two  places  to  be  connected,  as  the  towns  A  and  B 
on  the  plan,  may  both  be  situated  in  the  same  valley,  and  upon  the 
same  side  of  it;  that  is,  they  are  not  separated  from  each  other  by 
the  main  stream  which  drains  the  valley.  This  is  the  simplest 
case.  Second,  although  both  in  the  same  valley,  the  two  places 
may  be  on  opposite  sides  of  the  valley,  as  at  A  and  C,  being  sep- 
arated by  the  main  river.  Third,  the  two  places  may  be  situated 
in  different  valleys,  separated  by  an  intervening  ridge  of  ground 
more  or  less  elevated,  as  at  A  and  D.  In  laying  out  an  extensive 
line  of  road,  it  frequently  happens  that  all  these  cases  have  to  be 
dealt  with.  The  most  perfect  road  is  that  of  which  the  course 
is  perfectly  straight  and  the  surface  practically  level;  and,  all  other 
things  being  the  same,  the  best  road  is  that  which  answers  nearest 
to  this  description. 

Case  1.  Now,  in  the  first  case,  that  of  the  two  towns  situated 
on  the  same  side  of  the  main  valley,  there  are  two  methods  which 
may  be  pursued  in  forming  a  communication  between  them.  A 
road  following  the  direct  line  between  them,  shown  by  the  thick 
dotted  line  AB,  may  be  made,  or  a  line  may  be  adopted  which 
will  gradually  and  equally  incline  from  one  town  to  another,  sup- 
posing them  to  be  at  different  levels;  or,  if  they  are  on  the  same  level, 
the  line  should  keep  at  that  level  throughout  its  entire  course, 
following  all  the  sinuosities  and  curves  which  the  irregular  formation 
of  the  country  may  render  necessary  for  the  fulfillment  of  these 
conditions.  According  to  the  first  method,  a  level  or  uniformly 
inclined  road  might  be  made  from  one  to  the  other;  this  line 
would  cross  all  the  valleys  and  streams  which  run  down  to  the 


24  HIGHWAY  CONSTRUCTION 

main  river,  thus  necessitating  deep  cuttings,  heavy  embankments, 
and  numerous  bridges;  or  these  expensive  works  might  be  avoided 
by  following  the  sinuosities  of  the  valley.  When  the  sides  of  the 
main  valley  are  pierced  by  numerous  ravines  with  projecting  spurs 
and  ridges  intervening,  instead  of  following  the  sinuosities,  it  will 
be  found  better  to  make  a  nearly  straight  line  cutting  through 
the  projecting  points  in  such  a  way  that  the  material  excavated 
should  be  just  sufficient  to  fill  the  hollows. 

Of  all  these,  the  best  is  the  straight  uniformly  inclined  or  level 
road,  although  at  the  same  time  it  is  the  most  expensive.  If  the 
importance  of  the  traffic  passing  between  the  places  is  not  sufficient 
to  warrant  so  great  an  outlay,  it  will  become  a  matter  of  consider- 
ation whether  the  course  of  the  road  should  be  kept  straight,  its 
surface  being  made  to  undulate  with  the  natural  face  of  the  country; 
or  whether,  a  level  or  equally  inclined  line  being  adopted,  the  course 
of  the  road  should  be  made  to  deviate  from  the  direct  line  and  follow 
the  winding  course  which  such  a  condition  is  supposed  to  necessitate. 

Case  2.  In  the  second  case,  that  of  two  places  situated  on 
opposite  sides  of  the  same  valley,  there  is,  in  like  manner,  the  choice 
of  a  perfectly  straight  line  to  connect  them,  winch  would  probably 
require  a  big  embankment  if  the  road  were  kept  level;  or  steep 
inclines  if  it  followed  the  surface  of  the  country;  or  by  winding 
the  road,  it  might  be  carried  across  the  valley  at  a  higher  point, 
where,  if  the  level  road  be  taken,  the  embankment  would  not  be 
so  high,  or,  if  kept  on  the  surface,  the  inclination  would  be  reduced. 

Case  8.  In  the  third  case,  there  is,  in  like  manner,  the  alter- 
native of  carrying  the  road  across  the  intervening  ridge  in  a  perfectly 
straight  line,  or  of  deviating  it  to  the  right  and  left,  and  crossing 
the  ridge  at  a  point  where  the  elevation  is  less.  The  proper  deter- 
mination of  the  question  which  of  these  courses  is  the  best  under 
certain  circumstances  involves  a  consideration  of  the  comparative 
advantages  and  disadvantages  of  inclines  and  curves.  What 
additional  increase  in  the  length  of  the  road  would  be  equivalent 
to  a  given  inclined  plane  upon  it;  or  conversely,  what  inclination 
might  be  given  to  a  road  as  an  equivalent  to  a  given  decrease  in 
its  length?  To  satisfy  this  question,  the  comparative  force  required 
to  draw  different  vehicles  with  given  loads  must  be  known,  both 
upon  level  and  variously  inclined  roads. 


HIGHWAY  CONSTRUCTION  25 

The  route  which  will  give  the  most  general  satisfaction  consists 
in  following  the  valleys  as  much  as  possible  and  rising  afterward 
by  gentle  grades.  This  course  traverses  the  cultivated  lands, 
regions  studded  with  farmhouses  and  factories.  The  value  of  such 
a  line  is  much  more  considerable  than  that  of  a  route  by  the  ridges. 
The  watercourses  which  flow  down  to  the  main  valley  are,  it  is 
true,  crossed  where  they  are  the  largest  and  require  works  of  large 
dimensions,  but  also  they  are  fewer  in  number. 

Treatment  of  Intermediate  Towns.  Suppose  that  it  is  desired 
to  construct  a  road  between  two  distant  towns,  A  and  B,  Fig.  12, 
and  let  us  for  the  present  neglect  altogether  the  consideration  of 
the  physical  features  of  the  intervening  country,  assuming  that  it 
is  equally  favorable  whichever  line  we  select.  Now  at  first  sight 
it  would  appear  that,  under  such  circumstances,  a  perfectly  straight 
line  drawn  from  one  town  to 

the  other  would  be  the  best  xj\ 

that  could  be  chosen.  On 
more  careful  examination, 
however,  of  the  locality,  we 
may  find  that  there  is  a 
third  town  (7,  situated  some-  fi 

,  .  Fig.   12.     Diagram  Showing  Method  of  Deter- 

Wliat     On    One     Side      Or      the  mining  Line  of  Road  between  Successive 

Towns 

straight  line  which  we  have 

drawn  from  A  to  B;  and  although  our  primary  object  is  to  connect 
only  the  two  latter,  it  would  nevertheless  be  of  considerable  service 
if  all  three  towns  were  put  into  mutual  connection  with  each  other. 
This  may  be  effected  in  three  different  ways,  any  one  of  which 
might,  under  the  circumstances,  be  the  best.  In  the  first  place, 
we  might,  as  originally  suggested,  form  a  straight  road  from  A 
to  B,  and  in  a  similar  manner  two  other  straight  roads  from  A  to  C, 
and  from  B  to  C,  and  this  would  be  the  most  perfect  way  of  effecting 
the  object  in  view,  the  distance  between  any  of  the  two  towns  being 
reduced  to  the  least  distance  possible.  It  would,  however,  be 
attended  with  considerable  expense,  and  it  would  be  necessary  to 
construct  a  much  greater  length  of  road  than  according  to  the  second 
plan,  which  would  be  to  form,  as  before,  a  straight  road  from  A 
to  B,  and  from  C  to  construct  a  road  which  should  join  the  former 
at  a  point  D,  so  as  to  be  perpendicular  to  it  The  traffic  between 


\ 


26  HIGHWAY  CONSTRUCTION 

A  or  B  and  C  would  proceed  to  the  point  D  and  then  turn  off  to  C. 
With  this  arrangement,  while  the  length  of  the  roads  would  be 
very  materially  decreased,  only  a  slight  increase  would  be  occasioned 
in  the  distance  between  C  and  the  other  two  towns.  The  third 
method  would  be  to  form  only  the  two  roads  AC  and  CB,  in  which 
case  the  distance  between  A  and  B  would  be  somewhat  increased, 
while  that  between  A  and  C  or  B  and  C  would  be  diminished,  and 
the  total  length  of  road  to  be  constructed  also  would  be  lessened. 

As  a  general  rule  it  may  be  taken  that  the  last  of  these  methods 
is  the  best  and  most  convenient  for  the  public;  that  is  to  say,  that 
if  the  physical  character  of  the  country  does  not  determine  the 
course  of  the  road,  it  generally  will  be  found  best  not  to  adopt  a 
perfectly  straight  line,  but  to  vary  the  line  so  as  to  pass  through  all 
the  principal  towns  near  its  general  course. 

Treatment  of  Mountain  Roads.  The  location  of  roads  in 
mountainous  countries  presents  greater  difficulties  than  in  an  ordi- 
nary undulating  country;  the  same  latitude  in  adopting  undulating 
grades  and  choice  of  position  is  not  permissible,  for  the  maximum 
must  be  kept  before  the  eye  perpetually.  A  mountain  road  has 
to  be  constructed  on  the  maximum  grade  or  at  grades  closely  approx- 
imating it,  and  but  one  fixed  point  can  be  obtained  before  com- 
mencing the  survey,  and  that  is  the  lowest  pass  in  the  mountain 
range ;  from  this  point  the  survey  must  be  commenced.  The  reason 
for  this  is  that  the  lower  slopes  of  the  mountain  are  flatter  than 
those  at  their  summit;  they  cover  a  larger  area;  and  they  merge 
into  the  valley  in  diverse  undulations.  Consequently,  a  road  at  the 
foot  of  a  mountain  may  be  carried  at  will  in  the  desired  direction 
by  more  than  one  route,  while  at  the  top  of  a  mountain  range  any 
deviation  from  the  lowest  pass  involves  increased  length  of  line. 
The  engineer  having  less  command  of  the  ground,  owing  to  the 
reduced  area  he  has  to  deal  with  and  the  greater  abruptness  of  the 
slopes,  is  liable  to  be  frustrated  in  his  attempt  to  get  his  line  carried 
in  the  desired  direction. 

It  is  a  common  practice  to  run  a  mountain  survey  up  hill,  but 
this  should  be  avoided.  Whenever  an  acute-angled  zigzag  is  met 
with  on  a  mountain  road  near  the  summit,  the  inference  to  be  drawn 
is  that  the  line,  being  carried  up  hill,  on  reaching  the  summit  was 
too  low  and  the  zigzag  was  necessary  to  reach  the  desired  pass. 


HIGHWAY  CONSTRUCTION  27 

The  only  remedy  in  such  a  case  is  a  resurvey  beginning  at  the  summit 
and  running  down  hill.  This  method  requires  a  reversal  of  that 
usually  adopted.  The  grade  line  is  first  staked  out  and  its  horizontal 
location  surveyed  afterwards.  The  most  appropriate  instrument  for 
this  work  is  a  transit  with  a  vertical  circle  on  which  the  telescope 
may  be  set  to  the  angle  of  the  maximum  grade. 

Loss  of  Height.  "Loss  of  height"  is  to  be  carefully  avoided 
in  a  mountain  road.  By  loss  of  height  is  meant  an  intermediate 
rise  in  a  descending  grade.  If  a  descending  grade  is  interrupted  by 
the  introduction  of  an  unnecessary  ascent,  the  length  of  the  road 
will  be  increased,  over  that  due  to  the  continuous  grade,  by  the 
length  of  the  portion  of  the  road  intervening  between  the  summit 
of  the  rise  and  the  point  in  the  road  on  a  level  with  that  rise — a 
length  which  is  double  that  due  on  the  gradient  to  the  height  of 
the  rise.  For  example,  if  a  road  descending  a  mountain  rises,  at 
some  intermediate  point,  to  cross  over  a  ridge  or  spur,  and  the  height 
ascended  amounts  to  110  feet  before  the  descent  is  continued,  such 
a  road  would  be  just  one  mile  longer  than  if  the  descent  had  been 
uninterrupted;  for  110  feet  is  the  rise  due  to  a  half-mile  length  at 
a  slope  of  1  : 24. 

Water  on  Mountain  Roads.  -  Water  is  needed  by  the  workmen 
and  during  the  construction  of  the  road.  It  is  also  very  necessary 
for  the  traffic,  especially  during  hot  weather;  and  if  the  road  exceeds 
5  miles  in  length,  provision  should  be  made  to  have  the  water  either 
close  to  or  within  easy  reach  of  the  road.  With  a  little  ingenuity 
the  water  from  springs  above  the  road,  if  such  exist,  can  be  led 
down  to  drinking  fountains  for  men,  and  to  troughs  for  animals. 

In  a  tropical  country  it  would  be  a  matter  for  serious  con- 
sideration if  the  best  line  for  a  mountain  road  10  miles  in  length 
or  upwards,  but  without  water,  should,  not  be  abandoned  in  favor 
of  a  worse  line  with  a  water  supply  available. 

Halting  Places.  On  long  lines  of  mountain  roads,  halting 
places  should  be  provided  at  frequent  intervals. 

Alignment  of  Roads.  No  rule  can  be  laid  down  for  the  align- 
ment of  a  road — it  will  depend  upon  both  the  character  of  its  traffic 
and  the  "lay  of  the  land".  To  promote  economy  of  transportation, 
it  should  be  straight;  but,  if  straightness  is  obtained  at  the  expense 
of  easy  grades  that  might  have  been  obtained  by  deflections  and 


28 


HIGHWAY  CONSTRUCTION 


increase  in  length,  it  will  prove  very  expensive  to  the  community 
that  uses  it. 

The  curving  road  around  a  hill  often  may  be  no  longer  than 
the  straight  one  over  it,  for  the  latter  is  straight  only  with  reference 
to  the  horizontal  plane,  while  it  is  curved  as  to  the  vertical  plane; 
the  former  is  curved  as  to  the  horizontal  plane,  but  straight  as  to 
the  vertical  plane.  Both  lines  curve,  and  we  call  the  one  passing 
over  the  hill  straight  only  because  its  vertical  curvature  is  less 
apparent  to  our  eyes.  Excessive  crookedness  of  alignment  is  to 
be  avoided,  for  any  unnecessary  length  causes  a  constant  threefold 
waste :  first,  of  the  interest  of  the  capital  expended  in  making  that 
unnecessary  portion;  second,  of  the  ever  recurring  expense  of  repair- 
ing it;  and  third,  of  the  time  and  labor  employed  in  traveling  over  it. 


Fig.   13.     Diagram  Showing  Method  of  Laying  Out  Curve  in  Road 

Location  and  Construction  of  Curves.  Curves,  on  a  road  used 
exclusively  by  horse-drawn  traffic,  should  have  a  center  radius  of 
not  less  than  50  feet.  On  roads  used  by  both  horse-drawn  and 
motor- vehicle  traffic,  the  greatest  possible  radius  should  be  employed 
and  not  less  than  150  feet  at  the  inner  margin.  Curves  should 
not  be  placed  at  the  foot  of  a  steep  ascent;  and,  when  they  occur 
on  an  ascent,  the  grade  at  that  point  should  be  decreased  in  order 
to  compensate  for  the  additional  resistance  of  the  curve. 

Curves  may  be  either  circular  or  parabolic  in  form.  The 
latter  will  be  found  exceedingly  useful  for  joining  tangents  of  unequal 
length  and  for  following  contours;  when  the  curvature  is  least 
at  its  beginning  and  ending,  the  deviations  from  a  straight  line 


HIGHWAY  CONSTRUCTION  29 

are  less  strongly  marked  than  by  a  circular  arc.  The  connection 
between  a  circular  curve  and  its  tangents  should  be  made  by  a 
parabolic  arc. 

The  width  of  the  wheelway  on  curves  should  be  greater  than 
on  tangents;  the  position  in  which  the  additional  width  will  be  of 
the  greatest  service  to  the  traffic  is  at  the  entry  arcs,  as  shown  at 
A  and  A,  Fig.  13,  and  not  at  the  center  B  of  the  curve,  which  is 
the  point  commonly  widened.  The  minimum  radius  of  the  outer 
curve  to  provide  the  increased  width  may  be  determined  by  the 
formula 


in  which  R  is  radius  of  inner  curve;  W  is  width  of  road  on  tangents; 
w  is  width  of  vehicles;  and  I  is  maximum  length  of  vehicles,  including 
teams.  If  the  traffic  requires  it,  a  further  widening  may  be  obtained 
by  flattening  the  inner  curve  as  indicated  by  abc.  The  radius  of 
the  reversing  curves  should  be  not  less  than  15  feet. 

The  outer  half  of  the  wheelway  on  curves  used  by  fast  motor- 
vehicle  traffic  should  be  raised,  as  shown  in  Fig.  14,  the  amount 


I ~i.'".   14.     Diagram  Showing  Adjustment  of  Profile  on  Curves  for  Rapidly 
Moving  Motor  Vehicles 

of  elevation  being  4  inches  for  a  curve  of  150-foot  radius  and 
decreasing  to  2  inches  for  a  curve  of  300-foot  radius. 

The  approach  to  curves  should  afford  an  unobstructed  view 
for  at  least  300  feet,  and  obstructions  which  prevent  the  entire 
length  of  the  curve  from  being  seen  by  approaching  vehicles  should 
be  removed. 

Zigzags.  The  method  of  surmounting  a  height  by  a  series  of 
zigzags  or  by  a  series  of  reaches  with  practicable  curves  at  the 
turns,  is  objectionable  for  the  following  reasons : 

(1)  An  acute-angled  zigzag  obliges  the  traffic  to  reverse  its 
direction  without  affording  it  convenient  room  for  the  purpose. 
The  consequence  is  that  with  slow  traffic  a  single  train  of  vehicles 
is  brought  to  a  stand,  while  if  two  trains  of  vehicles,  traveling  in 
opposite  directions,  meet  at  the  zigzag,  a  block  ensues. 


30 


HIGHWAY  CONSTRUCTION 


00'99—OL  'Off— 
00-99— OI'SJL — 


-00  'PJL 21 'LL  — 

0'L+—00'9L—00'£9  — 


-02'2I+  -^O0'£0l — 02'SII^ 
2  3  & 


(2)  With    zigzags    little 
progress  is  made  toward  the 
ultimate    destination   of    the 
road;    height   is  surmounted, 
but  horizontal  distance  is  in- 
creased without  compensation. 

(3)  Zigzags  are  dangerous. 
Incase  of  a  runaway  down  hill, 
the  zigzag  must  prove  fatal. 

(4)  If  the  drainage  can- 
not be   carried  clear   of    the 
road  at  the  end  of  each  reach, 
it  must  be  carried  under  the 
road    in   one  reach,   only   to 
appear  again  at  the  next,  when 
a  second    bridge,   culvert,   or 
drain  will  be  required,  and  so 
on  at  the  other  reaches.    If  the 
drainage  can  be  carried  clear 
at  the    termination   of    each 
reach,  the  lengths  between  the 
curves  will  be  very  short,  en- 
tailing numerous  zigzag  curves, 
which  are  expensive    to  con- 
struct and  maintain. 

Details  after  Choosing  Route 

Final  Location.  With  the 
route  finally  determined  upon, 
it  must  be  located.  This 
consists  in  tracing  the  line, 
placing  a  stake  at  every  100 
feet  on  the  straight  portions 
and  at  every  50  or  25  feet  on 
the  curves.  At  the  tangent 
point  of  curves,  and  at  points 
of  compound  and  reverse 
curves,  a  larger  and  more  per- 


HIGHWAY  CONSTRUCTION  31 

manent  stake  should  be  placed.  Lest  those  stakes  should  be  dis- 
turbed in  the  process  of  construction,  their  exact  distance  from  several 
points  outside  of  the  ground  to  be  occupied  by  the  road  should  be 
carefully  measured  and  recorded  in  the  notebook,  so  that  they  may 
be  replaced.  The  stakes  above  referred  to  show  the  position  of 
the  center  line  of  the  road,  and  form  the  base  line  from  which  all 
operations  of  construction  are  carried  on.  Levels  are  taken  at  each 
stake,  and  cross  levels  are  taken  at  every  change  of  longitudinal  slope. 

Construction  Profile.  The  construction  or  working  profile  is 
made  from  the  levels  obtained  on  location.  It  should  be  drawn 
to  a  horizontal  scale  of  400  feet  to  the  inch  and  a  vertical  scale 
of  20  feet  to  the  inch.  Fig.  15  represents  a  portion  of  such  a  profile. 
The  figures  in  column  A  represent  the  elevation  of  the  ground  at 
every  100  feet,  or  where  a  stake  has  been  driven,  above  datum. 
The  figures  in  column  B  are  the  elevations  of  the  grade  above  datum. 
The  figures  in  column  C  indicate  the  depth  of  cut  or  height  of  fill; 
they  are  obtained  by  taking  the  difference  between  the  level  of 
the  road  and  the  level  of  the  surface  of  the  ground.  The  straight 
line  at  the  top  represents  the  grade  of  the  road;  the  upper  surface 
of  the  road  when  finished  would  be  somewhat  higher  than  this,  while 
the  given  line  represents,  what  is  termed  the  sub-grade  or  formation 
level.  All  the  dimensions  refer  to  the  formation  level,  to  which  the 
surface  of  the  ground  is  to  be  formed  to  receive  the  road  covering. 

At  all  changes  in  the  rate  of  inclination  of  the  grade  line  a 
heavier  vertical  line  should  be  drawn. 

Gradient.  The  grade  of  a  line  is  in  its  longitudinal  slope,  and 
is  designated  by  the  proportion  between  its  length  and  the  difference 
of  height  of  its  two  extremes.  The  ratio  of  these  two  qualities 
gives  it  its  name;  if  the  road  ascends  or  falls  one  foot  in  every  twenty 
feet  of  its  length,  it  is  said  to  have  a  grade  of  1  :  20,  or  a  5  per  cent 
grade.  Grades  are  of  two  kinds:  maximum  and  minimum.  The 
maximum  grade  is  the  steepest  which  is  to  be  permitted  and  which 
on  no  account  is  to  be  exceeded;  the  minimum  grade  is  the  least 
allowable  for  good  drainage.  Table  IX  gives  different  methods 
of  designating  grades. 

Determination  of  Gradients.  The  maximum  grade  is  fixed  by 
two  considerations:  the  one  relating  to  the  power  expended  in 
ascending,  and  the  other  to  the  acceleration  in  descending,  the 


32 


HIGHWAY  CONSTRUCTION 


TABLE  IX 

Methods  of  Designating  Grades 


AMERICAN  METHOD 
(ft.  per  100  ft.) 

ENGLISH  METHOD 

FEET  PER  MILE 

ANGLE  WITH  HORIZON 

i 

1     400 

13.2 

0°     8'  36" 

-£ 

1     200 

26.4 

0    17    11 

£ 

1     150 

39.6 

0    22    55 

1 

1     100 

52.8 

0    34    23 

U 

1       80 

66 

0    42    58 

i| 

1       66| 

79.2 

0    51    28 

if 

1       57J 

92.4 

1      0    51 

2 

1       50 

105.6 

186 

2| 

1       44| 

118.8 

1     17    39 

2£ 

1       40 

132 

1    25    57 

2| 

1       36| 

145.2 

1    34    22 

3 

1       33£ 

158.4 

1    43    08 

3| 

1       30| 

171.6 

1    51    42 

3| 

1       28^ 

184.8 

2      0    16 

3f 

1       26f 

198 

2      8    51 

4 

1       25 

211.2 

1     17    26 

4j 

1       23| 

224.4 

2    26    10 

4£ 

1       22| 

237.6 

2    34    36 

4| 

1       21 

250.8 

2    43    35 

5 

1       20 

284 

2    51    44 

6 

1       13f 

316.8 

3    26    12 

7 

1       14f 

369.6 

4      0    15 

8 

1       12i 

422.4 

4    34    26 

9 

i     Hi 

475.2 

5      8    31 

10 

1       10 

.      528 

5    42    37 

incline.  There  is  a  certain  inclination,  depending  upon  the  degree 
of  perfection  given  to  the  surface  of  the  road,  which  cannot  be 
exceeded  without  a  direct  loss  of  tractive  power.  This  inclination 
is  that,  on  which,  in  descending  at  a  uniform  speed,  the  traces 
slacken,  or  which  causes  the  vehicles  to  press  on  the  horses;  the 
limiting  inclination  within  which  this  effect  does  not  take  place  is 
the  angle  of  repose. 

Angle  of  Repose.  The  angle  of  repose  for  any  given  road  surface 
can  be  ascertained  easily  from  the  tractive  force  required  upon  a 
level  with  the  same  character  of  surface.  Thus  if  the  force  necessary 
on  a  level  to  overcome  the  resistance  of  the  load  is  ^V  of  its  weight, 
then  the  same  fraction  expresses  the  angle  of  repose  for  that  surface. 

On  all  inclines  less  steep  than  the  angle  of  repose,  a  certain 
amount  of  tractive  force  is  necessary  in  the  descent  as  well  as  in 
the  ascent,  and  the  mean  of  the  two  drawing  forces,  ascending  and 
descending,  is  equal  to  the  force  along  the  level  of  the  road.  Thus, 
on  such  inclines,  as  much  mechanical  force  is  gained  in  the  descent 


HIGHWAY  CONSTRUCTION  33 

as  is  lost  in  the  ascent.  From  this  it  might  be  inferred  that  when  a 
vehicle  passes  alternately  each  way  along  the  road,  no  real  loss  is 
occasioned  by  the  inclination  of  the  road;  which,  however,  is  not 
the  fact  with  animal  power,  for  wrhile  the  up  and  down  slopes  in 
the  ascending  journey  will  demand,  respectively,  a  greater  and  a 
less  number  of  horses  than  that  required  on  a  level  road,  no  actual 
compensation  for  this  fluctuation  can  be  made  in  the  descending 
journey.  On  inclines  which  are  more  steep  than  the  angle  of  repose, 
the  load  presses  on  the  horses  during  their  descent,  so  as  to  impede 
their  action,  and  their  power  is  expended  in  checking  the  descent 
of  the  load;  or  if  this  effect  be  prevented  by  the  use  of  any  form  of 
drag  or  brake,  then  the  power  expended  on  such  a  drag  or  brake 
corresponds  to  an  equal  quantity  of  mechanical  power  expended 
in  the  ascent,  for  which  no  equivalent  is  obtained  in  the  descent. 

Grade  Problems.  Maximum  Grade.  The  maximum  grade  for 
a  given  road  will  depend:  (1)  upon  the  class  of  traffic  that  will 
use  it,  whether  fast  and  light,  slow  and  heavy,  or  mixed,  consisting 
of  both  light  and  heavy;  (2)  upon  the  character  of  the  pavement 
adopted;  and  (3)  upon  the  question  of  cost  of  construction.  Econ- 
omy of  motive  power  and  low  cost  of  construction  are  antagonistic 
to  each  other,  and  the  engineer  will  have  to  weigh  the  two  in  the 
balance. 

For  fast  and  light  traffic  the  grades  should  not  exceed  2  per 
cent;  for  mixed  traffic  3  per  cent  may  be  adopted;  while  for  slow 
traffic  combined  with  economy  5  per  cent  should  not  be  exceeded. 
This  grade  is  practicable  but  not  convenient. 

Minimum  Grade.  From  the  previous  considerations  it  would 
appear  that  an  absolutely  level  road  was  the  one  to  be  sought  for, 
but  this  is  not  so;  there  is  a  minimum,  or  a  least  allowable  grade, 
of  which  the  road  must  not  fall  short,  as  well  as  a  maximum  one 
which  it  must  not  exceed.  If  the  road  were  perfectly  level  in  its 
longitudinal  direction,  its  surface  could  not  be  kept  free  from  water 
without  giving  it  so  great  a  rise  in  its  middle  as  would  expose  vehi- 
cles to  the  danger  of  overturning.  The  minimum  grade  commonly 
used  is  1  per  cent. 

Undulating  Grades.  From  the  fact  that  the  power  required 
to  move  a  load  at  a  given  velocity  on  a  level  road  is  decreased  on 
a  descending  grade  to  the  same  extent  it  is  increased  in  ascending 


34  HIGHWAY  CONSTRUCTION 

the  same  grade,  it  must  not  be  inferred  that  the  animal  force 
expended  in  passing  alternately  each  way  over  a  rising  and  falling 
road  will  gain  as  much  in  descending  the  several  inclines  as  it  will 
lose  in  ascending  them.  Such  is  not  the  case.  The  animal  force 
must  be  sufficient,  either  in  power  or  number,  to  draw  the  load  over 
the  level  portions  and  up  the  steepest  inclines  of  the  road,  and  in 
practice  no  reduction  in  the  number  of  horses  can  be  made  to  corre- 
spond with  the  decreased  power  required  in  descending  the  inclines. 

The  popular  theory  that  a  gently  undulating  road  is  less  fatiguing 
to  horses  than  one  which  is  perfectly  level  is  erroneous.  The  asser- 
tion that  the  alternations  of  ascent,  descent,  and  levels,  call  into 
play  different  muscles,  allowing  some  to  rest  while  others  are  exerted, 
and  thus  relieving  each  in  turn,  is  demonstrably  false,  and  con- 
tradicted by  the  anatomical  structure  of  the  horse.  Since  this 
doctrine  is  a  mere  popular  error,  it  should  be  rejected  utterly,  not 
only  because  it  is  false  in  itself,  but  still  more  because  it  encourages 
the  building  of  undulating  roads,  and  this  increases  the  labor  and 
cost  of  transportation  upon  them. 

Level  Stretches.  On  long  ascents  it  is  generally  recommended 
that  level  or  nearly  level  stretches  be  introduced  at  frequent  intervals 
in  order  to  rest  the  animals.  These  are  objectionable  when  they 
cause  loss  of  height,  and  animals  will  be  more  rested  by  halting 
and  unharnessing  for  half  an  hour  than  by  traveling  over  a  level 
portion.  The  only  case  which  justifies  the  introduction  of  levels 
into  an  ascending  road  is  where  such  levels  will  advance  the  road 
towards  its  objective  point;  where  this  is  the  case  there  will  be  no 
loss  of  either  length  or  height,  and  it  will  simply  be  exchanging  a 
level  road  below  for  a  level  road  above. 

Establishing  the  Grade.  When  the  profile  of  a  proposed  route 
has  been  made,  a  grade  line  is  drawn  upon  it  (usually  in  red)  in 
such  a  manner  as  to  follow  its  general  slope,  but  to  average  its 
irregular  elevations  and  depressions.  If  the  ratio  between  the  whole 
distance  and  the  height  of  the  line  is  less  than  the  maximum  grade 
intended  to  be  used,  this  line  will  be  satisfactory;  but  if  it  be  found 
steeper,  the  cut  or  the  length  of  the  line  will  have  to  be  increased. 
The  latter  is  generally  preferable. 

The  apex  or  meeting  point  of  all  curves  should  be  rounded  off 
by  a  vertical  curve,  as  shown  in  Fig.  16,  thus  slightly  changing 


HIGHWAY  CONSTRUCTION 


35 


the  grade  at  and  near  the  point  of  intersection.  A  vertical  curve 
rarely  need  extend  more  than  200  feet  each  way  from  that  point. 
Let  AB  and  BC  be  two  grades  in  profile  intersecting  at  station 
B,  and  let  A  and  C  be  the  adjacent  stations.  It  is  required  to  join 
the  grades  by  a  vertical  curve  extending  from  A  to  C.  Imagine  a 
chord  drawn  from  A  to  C.  The  elevation  of  the  middle  point  of 
the  chord  will  be  a  mean  of  the  elevations  of  the  grade  at  A  and  (7, 
and  one-half  of  the  difference  between  this  and  the  elevation  of 
the  grade  at  B  will  be  the  middle  ordinate  of  the  curve.  Hence 
we  have 

TIT      1  /grade  A+ grade  C  ,    D\ 

M  =  —l^     ^- graded  1 

in  which  M  equals  the  correction  in  elevation  for  the  point  B.     The 
correction  for  any  other  point  is  proportional  to  the  square  of  its 


I | J __L j 

Fig.  16.     Typical  Road  Section  Showing  Rounding  Off  of  Meeting  Points  of  Curves 

distance  from  A  to  C.  Thus,  assuming  the  distance  between 
successive  ordinates,  Fig.  16,  as  50  feet,  the  correction  ^4+25  is 
T&M;  at  ^4+50  it  is  \M;  at  A +7 '5  it  is  &M;  and  the  same  for 
corresponding  points  on  the  other  side  of  B.  The  corrections 
in  this  case  shown  are  subtractive,  since  M  is  negative.  They 
are  additional  when  M  is  positive,  and  the  curve  concave  upward. 

PRELIMINARY  ROAD  CONSTRUCTION  METHODS 
WIDTH  AND  TRANSVERSE  CONTOUR 

Width  of  Road.  A  road  should  be  wide  enough  to  accommodate 
the  traffic  for  which  it  is  intended,  and  should  comprise  a  wheelway 
for  vehicles  and  a  space  on  each  side  for  pedestrians. 

The  wheelway  of  country  highways  need  be  no  wider  than  is 
absolutely  necessary  to  accommodate  the  traffic  using  it;  in  many 


36  HIGHWAY  CONSTRUCTION 

places  a  track  wide  enough  for  a  single  team  is  all  that  is  necessary. 
But  the  breadth  of  the  land  appropriated  for  highway  purposes 
should  be  sufficient  to  provide  for  all  future  increase  of  traffic. 
The  wheelways  of  roads  in  rural  sections  should  be  double;  that  is, 
one  portion  paved  (preferably  the  center),  and  the  other  left  with 
the  natural  soil.  The  latter,  if  kept  in  repair,  will  be  preferred  by 
teamsters  for  at  least  one-half  the  year. 

The  minimum  width  of  the  paved  portion,  if  intended  to  carry 
two  lines  of  travel,  is  fixed  by  the  width  required  to  allow  two 
vehicles  to  pass  each  other  safely.  This  width  is  1 6  feet.  If  intended 
for  a  single  line  of  travel,  8  feet  is  sufficient,  but  suitable  turnouts 
must  be  provided  at  frequent  intervals.  The  most  economical 
width  for  any  roadway  is  some  multiple  of  eight.  Wide  roads 
are  the  best;  they  expose  a  larger  surface  to  the  drying  action  of 
the  sun  and  wind,  and  require  less  supervision  than  narrow  ones. 
Their  first  cost  is  greater  than  that  of  narrow  ones,  and  nearly 
in  the  ratio  of  the  increased  width. 

The  cost  of  maintaining  a  mile  of  road  depends  more  upon 
the  extent  of  the  traffic  than  upon  the  extent  of  its  surface,  and 
unless -extremes  be  taken,  the  same  quantity  of  material  will  be 
necessary  for  the  repair  of  roads,  either  wide  or  narrow,  which  are 
subjected  to  the  same  amount  of  traffic.  The  cost  of  spreading 
materials  over  the  wide  road  will  be  somewhat  greater,  but  the 
cost  of  the  materials  will  be  the  same.  On  narrow  roads  the  traffic 
being  confined  to  one  track,  will  wear  more  severely  than  if  spread 
over  a  wider  surface. 

The  width  of  land  appropriated  for  road  purposes  varies  in 
the  United  States  from  49  J  feet  to  66  feet;  in  England  and  France 
from  26  to  66  feet.  And  the  width  or  space  macadamized  is  also 
subject  to  variation;  in  the  United  States  the  average  width  is 
16  feet;  in  France  it  varies  between  16  and  22  feet;  in  Belgium 
8J  feet  seems  to  be  the  regular  width,  while  in  Austria,  from  14J 
to  26i  feet. 

Transverse  Contour.  The  centers  of  roadways  in  most  cases 
should  be  higher  than  the  sides,  the  object  being  to  facilitate  the 
flow  of  the  rain  water  to  the  gutters.  Where  a  good  surface  is 
maintained  a  very  moderate  amount  of  rise  is  sufficient  for  this 
purpose,  but  the  rise  should  bear  a  certain  proportion  to  the  width 


HIGHWAY  CONSTRUCTION 


37 


TABLE  X 

Proportionate  Rise  of  Center  to  Width  of  Carriageway  for 
Different  Road  Materials 


KIND  OF  SURFACE 

PROPORTIONS  OF  RISE  AT 
CENTER  TO  WIDTH  OF 
CARRIAGEWAY 

Earth 
Gravel 
Broken  stone 

1:40 
1:50 
1:60 

of  the  carriageway.  Earth  roads  require  the  most  and  asphalt 
the  least.  The  most  suitable  proportions  for  the  different  paving 
materials  is  shown  in  Table  X. 

Form  of  Contour.  All  authorities  agree  that  the  form  should 
be  convex,  but  they  differ  in  the  amount  and  form  of  the  convexity. 
Circular  arcs,  two  straight  lines  joined  by  a  circular  arc,  and  ellipses, 
all  have  their  advocates.  For  country  roads  a  curve  of  suitable 


Fig.  17.     Typical  Section  of  Road,  Showing  Contour 

convexity  may  be  obtained  as  follows:  At  J  of  the  width  from 
center  to  side,  make  the  rise  }  of  the  total  rise,  and  at  J  of  the  width 
make  the  rise  f  of  the  total,  Fig.  17. 

Excessive  height  and  convexity  of  cross  section  contract  the 
width  of  the  wheelway  by  concentrating  the  traffic  at  the  center, 
that  being  the  only  part  where  a  vehicle  can  run  upright.  The  force 
required  to  haul  vehicles  over  such  cross  sections  is  increased  because 
an  undue  proportion  of  the  load  is  thrown  upon  two  wheels  instead 
of  being  distributed  equally  over  the  four.  The  continual  tread 
of  horses'  feet  in  one  track  soon  forms  a  depression  which  holds 
water,  and  the  surface  is  not  so  dry  as  with  a  flat  section  which 
allows  the  traffic  to  distribute  itself  over  the  whole  width.  Sides 
formed  of  straight  lines  are  also  objectionable.  They  wear  hollow, 
retain  water,  and,  by  raising  the  center,  defeat  the  object  sought. 
The  required  convexity  should  be  obtained  by  rounding  the  forma- 
tion surface,  and  not  by  diminishing  the  thickness  of  the  covering 
at  the  sides. 


38  HIGHWAY  CONSTRUCTION 

Although  on  hillside  and  mountain  roads  it  is  generally  recom- 
mended that  the  surface  should  consist  of  a  single  slope  inclining 
inwards,  there  is  no  reason  for  or  advantage  gained  by  this  method. 
The  form -best  adapted  to  these  roads  is  the  same  as  for  a  road  under 
ordinary  conditions. 

With  a  roadway  raised  in  the  center  and  the  rain  water  draining 
off  to  gutters  on  each  side,  the  drainage  will  be  more  effectual  and 
speedy  than  if  the  drainage  of  the  outer  half  of  the  road  has  to  pass 
over  the  inner  half.  The  inner  half  of  such  a  road  is  usually  sub- 
jected to  more  traffic  than  the  outer  half.  If  formed  of  a  straight 
incline,  this  side  will  be  worn  hollow  and  retain  water.  The  inclined 
flat  section  never  can  be  properly  repaired  to  withstand  the  traffic. 
Consequently  it  never  can  be  kept  in  good  order,  no  matter  how 
constantly  it  may  be  mended.  It  is  always  below  par  and  when 
heavy  rain  falls  it  is  seriously  damaged. 

DRAINAGE 

Types  of  Drainage.  In  the  construction  of  roads,  drainage  is  of 
the  first  importance.  The  ability  of  earth  to  sustain  a  load  depends 
in  a  large  measure  upon  the  amount  of  moisture  retained  by  it. 
Most  earths  form  a  good  firm  foundation  so  long  as  they  are  kept 
dry,  but  when  wet  they  lose  their  sustaining  power,  becoming  soft 
and  incoherent. 

The  drainage  of  roadways  is  of  two  kinds,  viz,  subsurface  and 
surface.  .  The  first  provides  for  the  removal  of  the  underground 
water  found  in  the  body  of  the  road;  the  second  provides  for  the 
speedy  removal  of  all  water  falling  on  the  surface  of  the  road. 
Experience  has  shown  that  a  thorough  removal  of  the  underground 
water  is  of  the  utmost  importance  and  is  essential  to  'the  life  of  the 
road.  A  road  covering  placed  on  a  wet  undrained  bottom  will  be 
destroyed  by  both  water  and  frost,  and  will  always  be  troublesome 
and  expensive  to  maintain;  perfect  subsoil  drainage  is  a  necessity 
and  will,  be  found  economical  in  the  end  even  if  in  securing  it 
considerable  expense  is  required. 

Subsoil  Drainage 

The  methods  employed  for  securing  the  subsoil  drainage  must 
be  varied  according  to  the  character  of  natural  soil,  each  kind  of 
soil  requiring  different  treatment. 


HIGHWAY  CONSTRUCTION 


39 


Nature  of  Soils.  The  natural  soil  may  be  divided  into  the 
following  classes:  siliceous,  argillaceous,  and  calcareous;  rock, 
swamps,  and  morasses.  The  siliceous  and  calcareous  soils,  the 
sandy  loams  and  rock,  present  no  great  difficulty  in  securing  a 
dry  and  solid  foundation.  Ordinarily  they  are  not  retentive  of 
water  and  therefore  require  no  underdrains;  ditches  on  each  side 
of  the  road  will  generally  be  found  sufficient.  The  argillaceous 
soils  and  softer  marls  require  more  care;  they  retain  water  and  are 
difficult  to  compact,  except  at  the  surface;  and  they  are  very  unstable 
under  the  action  of  water  and  frost. 

Location  of  Drains.  The  removal  of  water  from  the  subsoil 
is  effected  by  drains  so  placed  as  to  intercept  the  underground 
circulation  of  the  water.  Regarding  the  best  location  for  the  drains 
to  accomplish  this,  three  cases  in  general  will  present  themselves: 

Marginal  Drains.  Where  the  subsoil  is  continually  wet  and 
without  a  well-defined  flow  of  water  from  either  side.  Under 


Fig.   18.     Typical  Road  Section  Showing  Marginal  Drains 

this  condition  marginal  drains,  as  shown  in  Fig.  18,  will  be  found 
satisfactory. 

Side  Drains.     Where  there  is  a  regular  flow  from  one  side  to 
the  other,  as  on  a  hillside  road,  a  single  drain  placed  on  the  side 


Fig.  19.     Typical  Road  Section  Showing  Side  Drains 

from  which  the  water  comes,  as  in  Fig.  19,  will  be  sufficient  usually. 

Center  and  Cross  Drains.    Where  the  subsoil  is  so  retentive 

of  water  as  to  require  a  system  of  drains  under  the  roadbed,  these 


40 


HIGHWAY  CONSTRUCTION 


drains  may  be  constructed  in  a  variety  of  ways.  The  simplest 
method  is  to  place  a  drain  under  the  center  of  the  roadway,  as  in 
Fig.  20,  connecting  it  at  intervals  by  cross  drains  with  drains  placed 
at  the  sides  which  discharge  into  the  natural  watercourses.  Where 


Fig.  20.     Typical  Road  Section  Showing  Center  Drain 

the  ground  is  level  or  has  but  a  slight  inclination,  the  cross  drains 
may  be  placed  at  right  angles  to  the  axis  of  the  road.  Where  there 
is  a  steep  grade  it  is  better  to  lay  the  cross  drains  in  the  form  of 
an  inverted  V  with  the  point  in  the  center  of  the  roadway  and 
directed  uphill. 

The  distance  apart  of  the  cross  drains  depends  upon  the  ease 
with  which  the  subsoil  yields  its  water.  In  porous  soils  the  drains 
will  prove  efficient  at  distances  of  from  30  to  40  feet;  in  retentive 
clay  the  spacing  may  range  from  10  to  20  feet. 

Proper  Fall  for  Drains.  The  fall  to  be  given  the  drains 
depends  upon  the  size  of  the  drain  and  the  amount  of  water  to  be 
carried  off.  It  is  not  advisable  to  employ  a  fall  greater  than  1  foot 
in  100  feet.  Too  great  a  fall  will  produce  a  swift  current  that  is 


Fig.  21.     "Blind"  Stone  Drain 


Fig.  22.     "Throat"  Stone  Drain 


liable  to  undermine  the  drain  as  well  as  to  choke  it  by  foreign 

matter,  which  a  less  rapid  stream  could  not  have  transported. 

Materials  Used   for    Drains.     The    materials    employed    for 

drains  are:     stone,  vitrified  clay  pipe,  porous  tile,  and  concrete. 


HIGHWAY  CONSTRUCTION  41 

Stone  Drains.  The  stone  drain  is  constructed  in  two  forms, 
shown  in  Figs.  21  and  22.  The  first  form,  called  a  "blind"  drain, 
consists  of  a  trench  excavated  to  the  required  depth  and  filled 
with  cobblestones  or  rounded  pebbles.  To  prevent  the  soil  from 
washing  in  and  choking  it,  the  larger  stones  are  covered  first  with 
a  layer  of  small  gravel,  and  then  with  a  layer  of  coarse  gravel,  by 
which  means  the  water  is  filtered  before  passing  into  the  porous 
mass  beneath.  Angular  stones  are  not  suitable  for  this  type  of 
drain.  The  second  form  of  stone  drain,  an  open  channel  called  a 
"throat",  is  formed  in  the  bottom  of  the  trench  with  rough  slabs 
of  stone,  and  the  trench  is  filled  in  the  same  manner  as  for  a 
blind  drain. 

Vitrified  Pipe  Drains.  Vitrified  pipe  drains  are  constructed 
by  placing  the  pipe  in  the  bottom  of  the  trench,  filling  the  hubs 
with  oakum  and  back-filling  the  trench  with  gravel,  broken  stone, 
or  a  mixture  of  these. 

Porous  Tile  Drains.     Porous  tile,  Fig.  23,  form  very  satisfactory 
drains.    They  carry  off  the  water  with  great  ease,  rarely  if  ever 
get  choked,  and  require  only  a  slight 
inclination  to  keep  the  water  moving 
through  them.     The  tile  have  plain 
ends  which  are  placed  in  contact  in 
the  trench   and   wrapped    with   tar 
paper  or   burlap.      They    are    sur- 
rounded and  covered  with  gravel  or 

.        .  Fig.  23.     Porous  Tile  Drain 

broken  stone  not  exceeding  1  inch  in 

size  for  a  depth  ranging  from  6  to  12  inches,  and  the  remaining 

depth  of  the  trench  is  filled  with  large  gravel  or  broken  stone. 

Concrete  Tile  Drains.  Tile  made  of  concrete  have  been  in 
satisfactory  service  for  several  decades.  They  are  generally  made  in 
lengths  of  one  foot  with  plain  ends  and. are  laid  in  the  same  manner 
as  the  porous  tile.  They  can  be  made  in  portable  machines  in  the 
vicinity  where  they  are  to  be  used — an  advantage  that  tends 
toward  low  first  cost. 

For  the  manufacture  of  concrete  tile  the  best  quality  of 
hydraulic  (Portland)  cement,  clean  sand,  and  fine  gravel  or  broken 
stone  should  be  used  in  the  proportion  of  one  part  cement,  two 
parts  sand,  and  four  parts  stone;  the  stone  should  contain  no  par- 


42 


HIGHWAY  CONSTRUCTION 


tides  exceeding  f  inch  in  size.  Sufficient  clean  water  should  be 
used  to  produce  a  "wet  mix",  which  should  be  poured  into  the 
molds  before  setting  begins  and  rammed  lightly.  After  setting  is 
completed,  the  tile  should  be  cured  for  about  90  days. 

Sizes  of  Drains.  The  size  of  the  drain  to  be  adopted  for  a 
given  situation  depends  upon  the  amount 
of  water  to  be  carried  and  the  fall  that  can 
be  given  the  drain.  These  two  factors  being 
given,  there  are  several  formulas  that  can 
be  used  to  determine  the  required  size.  But, 
in  the  subsoil  drainage  of  a  road,  the  amount 
of  water  to  be  moved  can  be  guessed  at 
only;  therefore,  experience  as  to  what  a 
drain  has  accomplished  in  a  given  locality 
is  a  better  guide  than  the  result  given  by 
any  formula.  Experience  shows  that  the 
least  practicable  size  is  4  inches.  The 
amount  of  water  to  be  moved  is  generally 
assumed  to  vary  between  J  inch  and  1  inch  per  acre,  per  24  hours, 
on  the  area  to  be  drained. 

Silt  Basins.  Silt  basins  should  be  constructed  at  all  junctions 
and  wherever  else  they  may  be  considered  necessary;  they  may  be 
made  from  a  single  6-inch  pipe,  Fig.  24,  or  constructed  of  brick 
masonry. 

Protection  of  Drain  Ends  from  Weather.  As  tile  drains  are 
more  liable  to  injury  from  frost  than  those  of  either  brick  or  stone, 


Fig.  24.     Typical  Construction 
for  Silt  Basin 


Fig.  25.     Proper  Method  of  Covering  Drain  Outlet 

their  ends  at  the  side  ditches  should  not  be  exposed  directly  to 
the  weather  in  very  cold  climates,  but  may  terminate  in  blind 


HIGHWAY  CONSTRUCTION  43 

drains,  or  a  few  lengths  of  vitrified  clay  pipe  reaching  under  the 
road  a  distance  of  about  3  or  4  feet  from  the  inner  slope  of  the  ditch. 

Drain  Outlets.  Drain  outlets  may  be  formed  by  building  a 
dwarf  wall  of  brick  or  stone,  whichever  is  the  cheapest  or  most 
convenient  in  the  locality.  The  outlet,  Fig.  25,  should  be  covered 
with  an  iron  grating  to  prevent  vermin  entering  the  drain  pipes 
and  building  nests,  thus  choking  the  waterway. 

Side  Ditches.  Side  ditches  are  provided  to  carry  away  the 
subsoil  water  from  the  base  of  the  road,  and  the  rain  water  which 
falls  upon  its  surface;  to  do  this  speedily  they  must  have  capacity 
and  inclination  proportionate  to  the  amount  of  water  reaching 
them.  The  width  of  the  bed  should  not  be  less  than  18  inches; 
the  depth  will  vary  with  circumstances,  but  should  be  such  that 
the  water  surface  shall  not  reach  the  subgrade,  but  remain  at  least 
12  inches  below  the  crown  of  the  road.  The  sides  should  slope 
at  least  1J  to  1. 

The  longitudinal  inclination  of  the  ditch  follows  the  configura- 
tion of  the  general  topography,  that  is,  the  lines  of  natural  drainage. 
When  the  latter  has  to  be  aided  artificially,  grades  of  from  1  in 
500  to  1  in  800  will  usually  answer. 

In  absorbent  soil  less  fall  is  sufficient,  and  in  certain  cases 
level  ditches  are  permissible.  The  slopes  of  the  ditches  must  be 
protected  where  the  grade  is  considerable.  This  can  be  accomplished 
by  sod  revetments,  riprapping,  or  paving. 

Surface  Drainage 

The  drainage  of  the  roadway  surface  depends  upon  the  preser- 
vation of  the  cross  section,  with  regular  and  uninterrupted  fall 
to  the  sides  and  without  hollows  or  ruts  in  which  the  water  can  lie, 
and  also  upon  the  longitudinal  fall  of  the  road.  If  this  is  not  suffi- 
cient the  road  becomes  flooded  during  heavy  rainstorms  and  melting 
snow,  and  is  considerably  damaged. 

Side  Ditches  and  Gutters.  The  removal  of  surface  water 
from  country  roads  may  be  effected  by  the  side  ditches,  into  which, 
when  there  are  no  sidewalks,  the  water  flows  directly.  When  there 
are  sidewalks,  gutters  are  formed  between  the  roadway  and  foot- 
path, and  the  water  is  conducted  from  these  gutters  into  the  side 
ditches  by  tile  piping  laid  under  the  walks  at  intervals  of  about 


44  HIGHWAY  CONSTRUCTION 

50  feet.  The  entrance  to  these  pipes  should  be  protected  against 
washing  by  a  rough  stone  paving.  In  the  case  of  covered  ditches 
under  the  footpath,  the  water  must  be  led  into  them  by  first  passing 
through  catch  basins.  These  are  small  masonry  vaults  covered 
with  iron  gratings  to  prevent  the  ingress  of  stones,  leaves,  etc. 
Connection  from  the  catch  basin  is  made  by  a  tile  pipe  about  6 
inches  in  diameter.  The  mouth  of  this  pipe  is  placed  a  few  feet 
above  the  bottom  of  the  catch  basin,  and  the  space  below  it  acts 
as  a  depository  for  the  silt  carried  by  the  water,  and  is  cleaned 
out  periodically.  The  catch  basins  may  be  placed  from  200  to 
300  feet  apart.  They  should  be  made  of  dimensions  sufficient 
to  convey  the  amount  of  water  which  is  liable  to  flow  into  them 
during  heavy  and  continuous  rains. 

If  on  inclines  the  velocity  of  the  water  is  greater  than  the  nature 
of  the  soil  will  withstand,  the  gutters  should  be  roughly  paved. 
In  all  cases,  the  slope  adjoining  the  footpath  should  be  covered 
with  sod.  A  velocity  of  30  feet  a  minute  will  not  disturb  clay 
with  sand  and  stone;  40  feet  per  minute  will  move  coarse  sand; 
60  feet  a  minute  will  move  gravel;  120  feet  a  minute  should  move 
round  pebbles  1  inch  in  diameter,  and  180  feet  a  minute  will  move 
angular  stones  If  inches  in  diameter. 

The  scour  in  the  gutters  on  inclines  may  be  prevented  by 
small  weirs  of  stones  or  wood  stick  fascines  constructed  by  the 
roadmen  at  a  nominal  cost.  At  junctions  and  crossroads  the 
gutters  and  side  ditches  require  careful  arrangement  so  that  the 
water  from  one  road  may  not  be  thrown  upon  another;  cross  drains 
and  culverts  will  be  required  at  such  places. 

Treatment  of  Springs  Found  in  Cuttings.  In  cuttings,  springs 
are  frequently  encountered  and  become  a  source  of  constant  danger 
to  the  stability  of  the  slopes.  In  such  cases  the  slope  should  be 
excavated  at  the  point  where  the  water  appears,  until,  if  possible, 
the  source  is  reached.  When  the  source  has  been  reached,  an  outlet 
is  provided  by  constructing  a  drain  and  connecting  it  with  the 
drain  at  the  roadside.  Sometimes  it  may  be  impossible  to  trace 
the  water  to  a  single  source,  the  whole  face  of  the  cutting  being 
saturated  for  some  distance.  In  such  cases  the  treatment  may 
be  difficult  and  expensive,  but  a  series  of  drains  may  be  run  up  the 
slope  to  such  height  as  will  tap  all  the  water  appearing. 


HIGHWAY  CONSTRUCTION  45 

In  cuttings,  the  ditch  at  the  toe  of  the  slope  is  liable  to  be 
filled  with  silt  carried  down  the  slope  by  rain;  and  where  this  might 
occur,  covered  drains  should  be  constructed. 

Drainage  for  Hillside  Roads.  On  hillside  or  mountain  roads 
catch- water  ditches  should  be  cut  on  the  mountain  side  above  the 
road,  to  cut  off  and  convey  the  drainage  of  the  ground  above  them 
to  the  neighboring  ravines.  The  size  of  these  ditches  will  be 
determined  by  the  amount  of  rainfall,  extent  of  drainage  from  the 
mountain  which  they  intercept,  and  by  the  distances  of  the  ravine 
watercourses  on  each  side. 

Inner  and  Outer  Road  Gutters.  The  inner  road  gutter  should 
be  of  dimensions  ample  to  carry  off  the  water  reaching  it;  when 
in  soil,  it  should  be  roughly  paved  with  stone.  When  paving  is 
not  absolutely  necessary,  but  is  desirable  to  arrest  the  scouring 
action  of  running  water  during  heavy  rains,  stone  weirs  may  be 
erected  across  the  gutter  at  convenient  intervals.  The  outer  gutter 
need  not  be  more  than  12  inches  wide  and  9  inches  deep.  The 
gutter  is  formed  by  a  depression  in  the  surface  of  the  road  close 
to  the  parapet  or  revetted  earthen  protection  mound.  The  drainage 
which  falls  into  this  gutter  is  led  off  through  the  parapet,  or  other 
roadside  protection,  at  frequent  intervals.  The  guard  stones  on 
the  outside  of  the  road  are  placed  in  and  across  the  gutter,  just 
below  the  drainage  holes,  so  as  to  turn  the  current  of  the  drainage 
into  these  holes  or  channels.  On  straight  reaches,  with  parapet 
protection,  drainage  holes  with  guard  stones  should  be  placed  every 
20  feet  apart.  Where  earthen  mounds  are  used,  and  it  may  not  be 
convenient  to  have  the  drainage  holes  or  channels  every  20  feet, 
the  guardstones  are  to  be  placed  in  advance  of  the  gutter  to  allow 
the  drainage  to  pass  behind  them.  This  drainage  is  either  to  be 
run  off  at  the  cross  drainage  of  the  road,  or  to  be  turned  off  as  before 
by  a  guard  stone  set  across  the  gutter. 

At  re-entering  turns,  where  the  outer  side  of  the  road  requires 
particular  protection,  guard  stones  should  be  placed  every  4  feet. 
As  all  re-entering  turns  should  be  protected  by  parapets,  the  drainage 
holes  through  them  may  be  placed  as  close  together  as  desired. 

Where  the  road  is  in  embankment  the  surface  water  must  be 
prevented  from  running  down  the  slopes  by  providing  ample  gutters 
suitably  connected  to  the  natural  watercourses. 


46  HIGHWAY  CONSTRUCTION 

Water  Breaks.  Water  breaks  to  turn  the  surface  drainage 
into  the  side  ditches,  should  not  be  constructed  on  improved  roads. 
They  increase  the  grade  and  are  an  impediment  to  convenient 
and  easy  travel.  WThere  it  is  necessary  that  water  should  cross 
the  road,  a  culvert  should  be  built. 

CULVERTS 

Functions  of  Culverts.  Culverts  are  necessary  for  carrying 
the  cross  streams  under  a  road,  and  also  for  conveying  the  surface 
water  collected  in  the  side  ditches  from  the  upper  side  to  that  side 
on  which  the  natural  watercourses  lie. 

Especial  care  is  required  to  provide  an  ample  way  for  the  water 
to  be  passed.  If  the  culvert  is  too  small,  it  is  liable  to  cause  a 
washout,  entailing  interruption  of  traffic  and  cost  of  repairs,  and 
possibly  may  cause  accidents  that  will  require  payment  of  large 
sums  for  damages.  On  the  other  hand,  if  the  culvert  is  made 
unnecessarily  large,  the  cost  of  construction  is  needlessly  increased. 

Factors  Considered  in  Design  of  Culverts.  The  area  of  water- 
way required  depends  upon  a  number  of  important  factors,  which 
will  be  discussed  briefly. 

Rate  of  Rainfall.  It  is  the  maximum  rate  of  rainfall  during 
the  severest  storms  which  is  required  in  this  connection.  This 
varies  greatly  in  different  sections  of  the  country. 

The  maximum  rainfall  as  shown  by  statistics  is  about  one  inch 
per  hour  (except  during  heavy  storms);  equal  to  3,630  cubic  feet 
per  acre.  Owing  to  various  causes,  not  more  than  50  to  75  per 
cent  of  this  amount  will  reach  the  culvert  within  the  same  hour. 

Inches  of  rainfall  X  3,630        =  cubic  feet  per  acre 

Inches  of  rainf all  X  2,323,200  =  cubic  feet  per  square  mile 

Kind  and  Condition  of  Soil.  The  amount  of  water  to  be  drained 
off  will  depend  upon  the  permeability  of  the  surface  of  the  ground, 
which  will  vary  greatly  with  the  kind  of  soil,  the  degree  of  saturation, 
the  condition  of  the  cultivation,  the  amount  of  vegetation,  etc. 

Character  and  Inclination  of  Surface.  The  rapidity  with  which  the 
water  will  reach  the  watercourse  depends  upon  whether  the  surface  is 
rough  or  smooth,  steep  or  flat,  barren  or  covered  with  vegetation,  etc. 

Condition  and  Inclination  of  Stream  Bed.  The  rapidity  with 
which  the  water  will  reach  the  culvert  depends  upon  whether  there 


HIGHWAY  CONSTRUCTION  47 

is  a  well-defined  and  unobstructed  channel  or  whether  the  water  finds 
its  way  in  a  broad,  thin  sheet.  If  the  watercourse  is  unobstructed  and 
has  a  considerable  inclination,  the  water  may  arrive  at  the  culvert 
nearly  as  rapidly  as  it  falls;  but  if  the  channel  is  obstructed,  the 
water  may  be  much  longer  in  passing  the  culvert  than  in  falling. 

Shape  of  Area  to  be  Drained  and  Position  of  Stream  Branches. 
The  area  of  waterway  depends  upon  the  amount  of  the  area  to 
be  drained ;  but  in  many  cases  the  shape  of  this  area  and  the  position 
of  the  branches  of  the  stream  are  of  more  importance  than  the 
amount  of  the  territory.  For  example,  if  the  area  is  long  and 
narrow,  the  water  from  the  lower  portion  may  pass  through  the 
culvert  before  that  from  the  upper  end  arrives;  or,  on  the  other 
hand,  if  the  upper  end  of  the  area  is  steeper  than  the  lower,  the  water 
from  the  former  may  arrive  simultaneously  with  that  from  the 
latter.  Again,  if  the  lower  part  of  the  area  is  supplied  better  with 
branches  than  the  upper  portion,  the  water  from  the  former  will 
be  carried  past  the  culvert  before  the  arrival  of  that  from  the  latter; 
or,  on  the  other  hand,  if  the  upper  part  is  supplied  better  with  branch 
watercourses  than  is  the  lowrer,  the  water  from  the  whole  area 
may  arrive  at  the  culvert  at  nearly  the  same  time.  In  large  areas 
the  shape  of  the  area  and  the  position  of  the  watercourses  are  very 
important  considerations. 

Mouth  of  Culvert  and  Inclination  of  Bed.  The  efficiency  of  a 
culvert  may  be  increased  very  materially  by  arranging  the  upper 
end  so  that  the  water  may  enter  into  it  without  being  retarded. 
The  discharging  capacity  of  a  culvert  can  be  increased  greatly  by 
increasing  the  inclination  of  its  bed,  provided  the  channel  below  will 
allow  the  water  to  flow  away  freely  after  having  passed  the  culvert. 

Provision  for  Discharge  of  Water  Under  Head.  The  discharging 
capacity  of  a  culvert  can  be  increased  greatly  by  allowing  the  water 
to  dam  up  above  it.  A  culvert  will  discharge  twice  as  much  under 
a  head  of  four  feet  as  under  a  head  of  one  foot.  This  can  be  done 
safely  only  with  a  well-constructed  culvert. 

The  determination  of  the  values  of  the  different  factors  entering 
into  the  problem  is  almost  wholly  a  matter  of  judgment.  An 
estimate  for  any  one  of  the  above  factors  is  liable  to  be  in  error 
from  100  to  200  per  cent,  or  even  more,  and  of  course  any  result 
deduced  from  such  data  must  be  very  uncertain.  Fortunately, 


48  HIGHWAY  CONSTRUCTION 

mathematical  exactness  is  not  required  by  the  problem  nor  warranted 
by  the  data.  The  question  is  not  one  of  10  or  20  per  cent  of  increase; 
for  if  a  2-foot  pipe  is  insufficient,  a  3-foot  pipe  probably  will  be 
the  next  size,  an  increase  of  225  per  cent;  and  if  a  6-foot  arch  culvert 
is  too  small,  an  8-foot  will  be  used,  an  increase  of  180  per  cent. 
The  real  question  is  whether  a  2-foot  pipe  or  an  8-foot  arch  culvert 
is  needed. 

Valuable  data  on  the  proper  size  of  any  particular  culvert  may 
be  obtained  as  follows:  (1)  by  observing  the  existing  openings  on 
the  same  stream;  (2)  by  measuring,  preferably  at  time  of  high 
water,  a  cross  section  of  the  stream  at  some  narrow  place;  and  (3) 
by  determining  the  height  of  high  water  as  indicated  by  drift  and 
debris,  and  from  the  evidence  of  the  inhabitants  of  the  neighborhood. 

On  mountain  roads,  or  roads  subjected  to  heavy  rainfall, 
culverts  of  ample  dimensions  should  be  provided  wherever  required, 
and  it  will  be  more  economical  to  construct  them  of  masonry.  In 
localities  where  boulders  and  debris  are  likely  to  be  washed  down 
during  wet  weather,  it  will  be  a  good  precaution  to  construct  catch 
pools  at  the  entrance  of  all  culverts  and  cross  drains  for  the  reception 
of  such  matter.  In  hard  soil  or  rock  these  catch  pools  will  be 
simple  well-like  excavations,  with  their  bottoms  two  or  three  feet 
below  the  entrance  sill  or  floor  of  the  culvert  or  drain.  Where 
the  soil  is  soft  they  should  be  lined  with  stone  laid  dry;  if  very  soft, 
with  masonry.  The  size  of  the  catch  pools  will  depend  upon  the 
width  of  the  drainage  works.  They  should  be  wide  enough  to 
prevent  the  drains  from  being  injured  by  falling  rocks  and  stones 
of  a  not  inordinate  size. 

The  use  of  catch  pools  obviates  the  necessity  of  building  culverts 
and  drains  at  an  angle  to  the  axis  of  the  road.  Oblique  structures 
are  objectionable,  as  being  longer  than  if  set  at  right  angles  and  by 
reason  of  the  acute-  and  obtuse-angled  terminations  to  their  piers, 
abutments,  and  coverings. 

Types  of  Culverts 

General  Classification.  Three  types  of  culverts  are  employed, 
namely:  pipe,  box,  and  arch.  The  pipe  culvert  is  employed  for 
small  streams,  in  sizes  from  12  to  24  inches.  Box  culverts  are 
employed  in  sizes  from  24  inches  up  to  8  feet.  Arch  culverts  are 


HIGHWAY  CONSTRUCTION  49 

used  for  spans  8  feet  and  over.  In  the  construction  of  culverts 
a  variety  of  materials  are  used.  Pipe  culverts  are  constructed 
of  earthenware  or  vitrified  clay,  cast  iron,  corrugated  steel,  brick, 
or  concrete;  box  and  arch  culverts  are  built  of  stone,  brick,  or  con- 
crete. Short  span  concrete  bridges  are  also  often  employed  as 
culverts.  The  type  of  culvert  and  the  material  to  be  used  are 
determined  in  some  cases  by  the  cost;  in  others  by  the  load  to  be 
supported,  as  where  the  depth  of  fill  over  the  culvert  is  considerable, 
or  where  a  large  area  of  waterway  is  required. 

Earthenware  Pipe  Culverts.  Construction.  In  laying  the  pipe 
the  bottom  of  the  trench  should  be  rounded  out  to  fit  the  lower 
half  of  the  body  of  the  pipe,  with  proper  depressions  for  the  sockets. 
If  the  ground  is  soft  or  sandy,  the  earth  should  be  rammed  carefully, 
but  solidly,  in  and  around  the  lower  part  of  the  pipe.  The  top 
surface  of  the  pipe,  as  a  rule,  never  should  be  less  than  18  inches 
below  the  surface  of  the  roadway,  but  there  are  many  cases  where 
pipes  have  stood,  for  several  years,  under  heavy  loads,  with  only 
8  to  12  inches  of  earth  over  them.  No  danger  from  frost  need  be 
apprehended,  provided  the  culverts  are  so  constructed  that  the 
water  is  carried  away  from  the  level  end.  Ordinary  soft  drain 
tiles  are  not  affected  in  the  least  by  the  expansion  of  frost  in  the 
earth  around  them. 

The  freezing  of  water  in  the  pipe,  particularly  if  more  than  half 
full,  is  liable  to  burst  it;  consequently  the  pipe  should  have  a  suffi- 
cient fall  to  drain  itself,  and  the  outside  should  be  so  low  that  there 
is  no  danger  of  backwaters  reaching  the  pipe.  If  properly  drained, 
there  is  no  danger  from  frost. 

Jointing.  In  many  cases,  perhaps  in  most,  the  joints  are 
not  calked.  If  this  is  not  done,  there  is  danger  of  the  water  being 
forced  out  of  the  joints  and  of  washing  away  the  soil  from  around 
the  pipe.  Even  if  the  danger  is  not  very  imminent,  the  joints 
of  the  larger  pipes,  at  least,  should  be  calked  with  hydraulic  cement, 
since  the  cost  is  very  small  compared  with  the  insurance  against 
damage  thereby  secured.  Sometimes  the  joints  are  calked  with 
clay.  Every  culvert  should  be  built  so  it  can  discharge  water 
under  a  head  without  damage  to  itself. 

Use  of  Bulkheads.    Although  often  omitted,  the  end  sections 
should  be  protected  with  a  masonry,  Fig.  26,  or  timber  bulkhead. 


50 


HIGHWAY  CONSTRUCTION 


The  foundation  of  the  bulkhead  should  be  deep  enough  not  to  be 
disturbed  by  frost.  In  constructing  the  end  wall,  it  is  well  to 
increase  the  fall  near  the  outlet  to  allow  for  a  possible  settlement 


Fig.  26.     Typical  Design  for  Masonry  Bulkhead 

of  the  interior  sections.  When  stone  and  brick  abutments  are  too 
expensive,  a  fair  substitute  can  be  made  by  setting  posts  in  the 
ground  and  spiking  plank  to  them.  When  planks  are  used,  it 
is  best  to  set  them  with  considerable  inclination  towards  the  road- 
bed to  prevent  their  being  crowded  outward  by  the  pressure  of  the 
embankment.  The  upper  end  of  the  culvert  should  be  so  protected 


1 
I 

Fig.  27.     Section  Showing  Typical  Layout  for  Double  Pipe  Culvert 

that  the  water  will  not  readily  find  its  way  along  the  outside  of  the 

pipes,  in  case  the  mouth  of  the  culvert  should  become  submerged. 

When  the  capacity  of  one  pipe  is  not  sufficient,  two  or  more 

may  be  laid  side  by  side  as  shown  in  Fig.  27.     Although  the  two 


HIGHWAY  CONSTRUCTION 


51 


small  pipes  do  not  have  as  much  discharging  capacity  as  a  single 
large  one  of  equal  cross  section,  yet  there  is  an  advantage  in  laying 
two  small  ones  side  by  side,  since  the  water  need  not  rise  so  high 
to  utilize  the  full  capacity  of  the  two  pipes  as  would  be  necessary 
to  discharge  itself  through  a  single  one  of  large  size. 

Iron  Pipe  Culverts.  During  recent  years  iron  pipe,  Fig.  28, 
has  been  used  for  culverts  on  many  prominent  railroads,  and  may 
be  used  on  roads  in  sections  where  other  materials  are  unavailable. 

In  constructing  a  culvert  with  cast-iron  pipe  the  points  requiring 
particular  attention  are:  (1)  tamping  the  soil  tightly  around  the 
pipe  to  prevent  the  water  from  forming  a  channel  along  the  outside; 


Fig.  28.     Section  Showing  Construction  of  Iron  Pipe  Culvert 

and  (2)  protecting  the  ends  by  suitable  head  walls  and,  when  neces- 
sary, laying  riprap  at  the  lower  end.  The  amount  of  masonry 
required  for  the  end  walls  depends  upon  the  relative  width  of  the 
embankment  and  the  number  of  sections  of  pipe  used.  For  example, 
if  the  embankment  is,  say,  40  feet  wide  at  the  base,  the  culvert 
may  consist  of  three  12-foot  lengths  of  pipe  and  a  light  end  wall 
near  the  toe  of  the  bank;  but  if  the  embankment  is,  say,  32  feet 
wide,  the  culvert  may  consist  of  two  12-foot  lengths  of  pipe  and  a 
comparatively  heavy  end  wall  well  back  from  the  toe  of  the  bank. 
The  smaller  sizes  of  pipe  usually  come  in  12-foot  lengths,  but  some- 
times a  few  6-foot  lengths  are  included  for  use  in  adjusting  the 
length  of  the  culvert  to  the  width  of  the  bank.  The  larger  sizes 
are  generally  6  feet  long. 


52  HIGHWAY  CONSTRUCTION 

Box  Culverts.  Box  culverts,  Fig.  29,  consist  of  two  side  walls 
with  a  flat  deck.  When  stone  is  used,  they  are  generally  built  of 
dry  rubble  masonry.  The  walls  should  be  well  founded  at  about 
42  inches  below  the  bed  of  the  stream.  The  thickness  of  the  walls 
varies  according  to  the  height.  The  wings  are  formed  by  extending 
the  walls  out  straight  and  stepping  them  down.  The  deck  may  be 
made  of  stone  slabs  or  reinforced  concrete;  with  the  latter  it  is 
possible  to  use  wider  spans  than  with  stone  slabs.  Where  the  force 
of  the  stream  is  sufficient  to  scour  the  bed,  it  will  be  necessary  to 


End  View  and  Section 


Lonqiludinal  Section 
Fig.  29.     Typical  Design  of  Concrete  Box  Culvert 

pave  it  with  stone  or  concrete.  When  reinforced  concrete  is  used 
instead  of  stone,  the  side  walls  are  made  from  4  to  8  inches  thick, 
depending  upon  the  height.  Where  it  is  not  necessary  to  pave 
the  stream  bed,  the  walls  are  carried  down  about  2  feet  below  the 
bed,  and  founded  upon  a  footing  9  to  12  inches  thick  and  sufficiently 
wide  to  secure  ample  area  of  the  soil  to  support  the  load.  Where 
scouring  of  the  bed  is  liable  to  occur,  a  concrete  bottom  is  con- 
structed throughout  the  entire  width  and  length  of  the  culvert, 
and  the  side  walls  are  founded  on  it;  if  necessary,  a  cut-off  wall 


HIGHWAY  CONSTRUCTION 


53 


is  constructed  across  each  end  to  a  depth  of  about  2  feet  below 
the  bottom. 

Arch  Culverts.  The  arch  form  of  culvert  is  more  costly  than 
the  other  forms,  but  it  is  often  preferred  on  account  of  its  appearance, 
Fig.  30.  When  masonry  and  plain  concrete  are  used,  very  heavy 
abutments  are  required  in  order  that  no  movement  can  take  place 
under  a  live  load,  to  cause  bending  moments  in  the  arch.  In  design- 
ing reinforced-concrete  arches,  bending  is  provided  for  by  consider- 


Section  and  End  Vie 


Side  Elevation. 


Fig.  30.     Design  for  Arch  Culvert 


ing  the  arch  as  a  curved  beam,  with  a  consequent  reduction  in  the 
weight  of  the  abutments. 

Short  Span  Bridges  Used  as  Culverts.  Three  types  of  rein- 
forced-concrete  bridges  are  employed  for  short  spans:  (a)  the  flat 
slab;  (6)  the  T-beam;  (c)  the  steel  I-beam  incased  in  concrete, 
Fig.  31.  The  length  of  span  over  which  reinforced  slabs  may  be 
built  with  safety  depends  upon  the  load  to  be  carried;  under  normal 
conditions  the  maximum  span  is  12  feet.  The  thickness  of  the  slab 
for  a  span  2  feet  should  be  not  less  than  6  inches  and  should  increase 
with  increase  of  span.  The  slabs  are  reinforced  with  steel  bars, 
expanded  metal,  or  other  forms  of  reinforcing  metal;  the  cross- 


54 


HIGHWAY  CONSTRUCTION 


sectional  area  of  the  reinforcing  steel  required  is  about  1  per  cent 
of  that  of  the  slab.* 

The  T-beam  type  is  practicable  for  spans  from  12  to  30  feet. 
The  I-beam  type  may  be  used  for  all  spans  up  to  30  feet.  In  this 
type,  the  I-beam  is  designed  to  transmit  the  load  to  the  abutments, 
while  the  reinforced-concrete  floor  transmits  the  load  to  the  I-beams. 
This  type  of  construction  is  noted  for  its  safety  and  ability  to  with- 
stand severe  and  unfavorable  conditions,  such  as  the  settlement 
of  the  abutments,  which  may  cause  rupture  of  the  concrete.  The 


wr 

5eclion  of  Flal  Slab  CL 


Reinforcement^ 


Section  Through  Top  For  T  Beam  Culvert 
(Reinforcement  ,^.rr^.Trr..^7T..r^:^r-,..^,.r_  Steel  I- 


•Section  Through  Top  For  I  Beam 
Fig.  31.     Sections  of  Typical  Short  Span  Concrete  Bridges  Used  as  Culverts 

I-beam  may  or  may  not  be  incased  in  the  concrete;  the  object  sought 
in  so  doing  is  to  protect  it  from  rust.  This  may  be  accomplished 
also  by  painting,  but  as  this  needs  to  be  repeated  frequently  and 
as  there  is  a  possibility  that  it  will  not  be  done,  it  is  better  to  incase 
the  beams  in  the  concrete  during  construction,  and  so  insure  their 
permanent  protection.  This  type  also  admits  of  arch  construction 
between  the  beams  for  the  floor  system,  thus  decreasing  the  depth 
required  for  the  floor;  this  feature  may  be  of  value  in  locations  where 
the  area  of  the  waterway  or  the  "head  room"  is  a  controlling  factor. 

"The  theory  of  design  of  concrete  bridges  and  culverts  is  discussed  in  Masonry  and  Rein- 
forced Concrete,  Part  III. 


HIGHWAY  CONSTRUCTION  55 

EARTHWORK 

The  term  "earthwork"  is  applied  to  all  the  operations  per- 
formed in  the  making  of  excavations  and  embankments.  In  its 
widest  sense  it  comprehends  work  in  rock  as  well  as  in  the  looser 
materials  of  the  earth's  crust. 

Balancing  Cuts  and  Fills.  In  the  construction  of  new  roads, 
the  formation  of  the  roadbed  consists  in  bringing  the  surface  of  the 
ground  to  the  adopted  grade.  This  grade  should  be  established  so  as 
to  reduce  the  earthwork  to  the  least  possible  amount,  both  to  render 
the  cost  of  construction  low,  and  to  avoid  unnecessarily  marring 
the  appearance  of  the  country  in  the  vicinity  of  the  road.  The 
most  desirable  position  of  the  grade  line  is  usually  that  which  makes 
the  amounts  of  cutting  and  filling  equal  to  each  other,  for  any  sur- 
plus embankment  over  cutting  must  be  made  up  by  borrowing, 
%and  surplus  cutting  must  be  wasted;  both  of  these  operations 
involving  additional  cost  for  labor  and  land. 

Side  Slopes.  Inclination.  The  proper  inclination  for  the 
side  slopes  of  cutting  and  embankments  depends  upon  the  nature 
of  the  soil,  the  action  of  the  atmosphere,  and  the  action  of  internal 
moisture  upon  it.  For  economy  the  inclination  should  be  as  steep 
as  the  nature  of  the  soil  will  permit. 

The  usual  slopes  in  cuttings  are: 

Solid  rock  £          to  1 

Earth  and  gravel  1£        to  1 

Clay  3  or  6  to  1 

Fine  sand  2  or  3  to  1 

The  slopes  of  embankment  are  usually  made  1J  to  1. 

Form  of  Slopes.  The  natural,  strongest,  and  ultimate  form 
of  earth  slopes  is  a  concave  curve,  in  which  the  flattest  portion 
is  at  the  bottom.  This  form  is  very  rarely  given  to  the  slopes  in 
constructing  them;  in  fact,  the  reverse  is  often  the  case,  the  slopes 
being  made  convex,  thus  saving  excavation  by  the  contractor 
and  inviting  slips. 

In  cuttings  exceeding  10  feet  in  depth  the  forming  of  concave 
slopes  will  aid  materially  in  preventing  slips,  and  in  any  case  they 
will  reduce  the  amount  of  material  which  eventually  will  have  to 
be  removed  when  cleaning  up.  Straight  or  convex  slopes  will 
continue  to  slip  until  the  natural  form  is  attained. 


56  HIGHWAY  CONSTRUCTION 

A  revetment  or  retaining  wall  at  the  base  of  a  slope  will  save 
excavation. 

In  excavations  of  considerable  depth,  and  particularly  in  soils 
liable  to  slips,  the  slope  may  be  formed  in  terraces,  the  horizontal 
offsets  or  benches  being  made  a  few  feet  in  width  with  a  ditch  on 


Fig.  32.     Section  Showing  Correct  Slopes  of  Embankments 

the  inner  side  to  receive  the  surface  water  from  the  portion  of  the 
side  slope  above  them.    These  benches  catch  and  retain  earth 


Fig.  33.     Section  Showing  Correct  Slopes  of  Excavations 

that  may  fall  from  the  slopes  above  them.  The  correct  forms  for 
the  slopes  of  embankment  and  excavation  are  shown  in  Figs.  32 
and  33. 

Covering  of  Slopes.  It  is  not  usual  to  employ  any  artificial 
means  to  protect  the  surface  of  the  side  slopes  from  the  action  of 
the  weather;  but  it  is  a  precaution  which  in  the  end  will  save  much 
labor  and  expense  in  keeping  the  roadways  in  good  order.  The 
simplest  means  which  can  be  used  for  this  purpose  consist  in  cover- 
ing the  slopes  with  good  sods,  or  else  with  a  layer  of  vegetable 
mold  about  four  inches  thick,  carefully  laid  and  sown  with  grass 
seed.  These  means  are  amply  sufficient  to  protect  the  side  slopes 
from  injury  when  they  are  not  exposed  to  any  other  cause  of 
deterioration  than  the  wash  of  the  rain  and  the  action  of  frost 
on  the  ordinary  moisture  retained  by  the  soil. 

A  covering  of  brushwood  or  a  thatch  of  straw  may  also  be  used 
with  good  effect;  but  from  their  perishable  nature  they  will  require 
frequent  renewal  and  repairs. 

Where  stone  is  abundant  a  small  wall  of  stone  laid  dry  may  be 
constructed  at  the  foot  of  the  slopes  to  prevent  any  wash  from  them 
being  carried  into  the  ditches. 


HIGHWAY  CONSTRUCTION  57 

Shrinkage  of  Earthwork.  All  materials  when  excavated  in- 
crease in  bulk,  but  after  being  deposited  in  banks  subside  or  shrink 
(rock  excepted)  until  they  occupy  less  space  than  in  the  pit  from 
which  excavated. 

Rock,  on  the  other  hand,  increases  in  volume  by  being  broken 
up,  and  does  not  settle  again  into  less  than  its  original  bulk.  The 
increase  may  be  taken  at  50  per  cent. 

The  shrinkage  in  the  different  materials  is  about  as  follows: 

Gravel  8  per  cent 

Gravel  and  sand  9  per  cent 

Clay  and  clay  earths  10  per  cent 

Loam  and  light  sandy  earths  12  per  cent 

Loose  vegetable  soil  15  per  cent 

Puddled  clay  25  per  cent 

Thus  an  excavation  of  loam  measuring  1000  cubic  yards  will 
form  only  about  880  cubic  yards  of  embankment,  or  an  embankment 
of  1000  cubic  yards  will  require  about  1120  cubic  yards,  measured 
in  excavation,  to  make  it.  A  rock  excavation  measuring  1000  yards 
will  make  from  1500  to  1700  cubic  yards  of  embankment,  depending 
upon  the  size  of  the  fragments. 

The  lineal  settlement  of  earth  embankments  will  be  about  in 
the  ratio  given  above;  therefore  either  the  contractor  should  be 
instructed,  in  setting  his  poles  to  guide  him  as  to  the  height  of  grade 
on  an  earth  embankment,  to  add  the  required  percentage  to  the 
fill  marked  on  the  stakes,  or  the  percentage  may  be  included  in 
the  fill  marked  on  the  stakes.  In  rock  embankments  this  is  not 
necessary. 

Classification  of  Earthwork.  Excavation  is  usually  classified 
as  earth,  hardpan,  loose  rock,  or  solid  rock.  For  each  of  these 
classes  a  specific  price  is  usually  agreed  upon,  and  an  extra  allowance 
is  sometimes  made  when  the  haul,  or  distance  to  which  the  excavated 
material  is  moved,  exceeds  a  given  amount. 

The  characteristics  which  determine  the  classes  to  which  a 
given  material  belongs  are  usually  described  with  clearness  in  the 
specifications,  as : 

Earth,  to  include  loam,  clay,  sand,  and  loose  gravel. 

Hardpan,  to  include  cemented  gravel,  slate,  cobbles,  and 
boulders  containing  less  than  1  cubic  foot,  and  all  other  material 
of  an  earthy  nature,  however  compact  it  may  be. 


58  HIGHWAY  CONSTRUCTION 

Loose  rockt  to  include  shale,  decomposed  rock,  boulders,  and 
detached  masses  of  rock  containing  not  less  than  3  cubic  feet,  and 
all  other  material  of  a  rock  nature  which  may  be  loosened  with  a 
pick,  although  blasting  may  be  resorted  to  in  order  to  expedite  the 
work. 

Solid  rock,  to  include  all  rock  found  in  place  in  ledges  and 
masses,  or  boulders  measuring  more  than  3  cubic  feet,  and  which 
can  only  be  removed  by  blasting. 

Prosecution  of  Earthwork.  No  general  rule  can  be  laid  down 
for  the  exact  method  of  carrying  on  an  excavation  and  disposing 
of  the  excavated  material.  The  operation  in  each  case  can  be 
determined  only  by  the  requirements  of  the  contract,  character 
of  the  material,  magnitude  of  the  work,  length  of  haul,  etc. 

Methods  of  Forming  Embankments.  General  Case.  Where 
embankments  are  to  be  formed  less  than  2  feet  in  height,  all  stumps, 
weeds,  etc.,  should  be  removed  from  the  space  to  be  occupied  by 
the  embankment.  For  embankments  exceeding  2  feet  in  height 
stumps  need  only  be  close  cut.  Weeds  and  brush,  however,  ought 
to  be  removed  and  if  the  surface  is  covered  with  grass  sod,  it  is 
advisable  to  plow  a  furrow  at  the  toe  of  the  slope.  Where  a  cut 
passes  into  a  fill  all  the  vegetable  matter  should  be  removed  from 
the  surface  before  placing  the  fill.  The  site  of  the  bank  should 
be  examined  carefully  and  all  deposits  of  soft,  compressible  matter 
removed.  When  a  bank  is  to  be  made  over  a  swamp  or  marsh, 
the  site  should  be  drained  thoroughly,  and  if  possible  the  fill  should 
be  started  on  hard  bottom. 

Perfect  stability  is  the  object  aimed  at,  and  all  precautions 
necessary  to  this  end  should  be  taken.  Embankments  should  be 
built  in  successive  layers:  banks  2  feet  and  under  in  layers  from  6 
inches  to  1  foot;  heavier  banks  in  layers  2  and  3  feet  thick.  The 
horses  and  vehicles  conveying  the  materials  should  be  required  to 
pass  over  the  bank  for  the  purpose  of  consolidating  it,  and  care 
should  be  taken  to  have  the  layers  dip  towards  the  center.  Embank- 
ments which  have  been  first  built  up  in  the  center,  and  after- 
wards widened  by  dumping  the  earth  over  the  sides,  should  never 
be  allowed. 

Embankments  on  Hillsides.  When  the  axis  of  the  road  is 
laid  out  on  the  side  slope  of  a  hill,  and  the  road  is  formed  partly 


HIGHWAY  CONSTRUCTION  59 

by  excavating  and  partly  by  embanking,  the  usual  and  most  simple 
method  is  to  extend  out  the  embankment  gradually  along  the  whole 
line  of  the  excavation.  This  method  is  insecure;  the  excavated 
material  if  simply  deposited  on  the  natural  slope  is  liable  to  slip, 
and  no  pains  should  be  spared  to  give  it  a  secure  hold,  particularly 
at  the  toe  of  the  slope.  The  natural  surface  of  the  slope  should  be 
cut  into  steps,  as  shown  in  Fig.  34.  The  dotted  line  AB  represents 
the  natural  surface  of  the  ground,  CEB  the  excavation,  and  ADC 
the  embankment,  resting  on  steps  which  have  been  cut  between  A 
and  C.  The  best  position  for  these  steps  is  perpendicular  to  the 
axis  of  greatest  pressure.  If  AD  is  inclined  at  the  angle  of  repose 
of  the  material,  the  steps  near  A  should  be  inclined  in  the  oppo- 
site direction  to  AD,  and  at  an  angle  of  nearly  90  degrees  thereto, 


Fig.  34.    Section  of  Embankment  Showing  Reinforcing  by  Means  of  Steps 

while  the  steps  near  C  may  be  level.  If  stone  is  abundant  in  the 
locality,  the  toe  of  the  slope  may  be  further  secured  by  a  dry  wall 
of  stone. 

On  hillsides  of  great  inclination  the  above  method  of  construc- 
tion will  not  be  sufficiently  secure;  retaining  walls  of  stone  must 
be  substituted  for  the  side  slopes  of  both  the  excavations  and  embank- 
ments. These  walls  may  be  made  of  stone  laid  dry,  when  stone 
can  be  procured  in  blocks  of  sufficient  size  to  render  this  kind  of 
construction  of  sufficient  stability  to  resist  the  pressure  of  the 
earth.  When  the  stones  laid  dry  do  not  offer  this  security,  they 
must  be  laid  in  mortar.  The  wall  which  forms  the  slope  of  the 
excavation  should  be  carried  up  as  high  as  the  natural  surface 
of  the  ground.  Unless  the  material  is  such  that  the  slope  may  be 
safely  formed  into  steps  or  benches,  as  shown  in  Fig.  34,  the  wall 
that  sustains  the  embankment  should  be  built  up  to  the  surface 


60 


HIGHWAY  CONSTRUCTION 


of  the  roadway,  and  a  parapet  wall  or  fence  raised  upon  it,  to  protect 
pedestrians  against  accident,  Fig.  35. 

For  the  formula  for  calculating  the  dimensions  of  retaining 
walls  see  Instruction  Paper  on  Masonry  and  Reinforced  Concrete, 
Part  III. 

Treatment  of  Roadways  on  Rock  Slopes.  On  rock  slopes,  when 
the  inclination  of  the  natural  surface  is  not  greater  than  1  on  the 
vertical  to  2  on  the  base,  the  road  may  be  constructed  partly  in 
excavation  and  partly  in  embankment  in  the  usual  manner,  or 
by  cutting  the  face  of  the  slope  into  horizontal  steps  with  vertical 
faces,  and  building  up  the  embankment  in  the  form  of  a  solid  stone 
wall  in  horizontal  courses,  laid  either  dry  or  in  mortar.  Care  is 
required  in  proportioning  the  steps,  as  all  attempts  to  lessen  the 


Fig.  35.     Reinforcing  Roadway  by  Parapet  Wall  or  Fence 

quantity  of  excavation,  by  increasing  the  number  and  diminishing 
the  width  of  the  steps,  require  additional  precautions  against  settle- 
ment in  the  built-up  portion  of  the  roadway. 

When  the  rock  slope  has  a  greater  inclination  than  1:2  the 
whole  of  the  roadway  should  be  in  excavation. 

In  some  localities  roads  have  been  constructed  along  the  face 
of  nearly  perpendicular  cliffs,  on  timber  frameworks  consisting  of 
horizontal  beams  firmly  fixed  at  one  end  by  being  let  into  holes 
drilled  in  the  rock,  the  other  end  being  supported  by  an  inclined 
strut  resting  against  the  rock  in  a  shoulder  cut  to  receive  it.  There 
are  also  examples  of  similar  platforms  suspended  instead  of  being 

supported. 

Tools  for  Construction  Work 

Picks.  Picks  are  made  in  various  styles,  according  to  the  class 
of  material  in  which  they  are  to  be  used.  Fig.  36  shows  the  form 


HIGHWAY  CONSTRUCTION 


61' 


usually  employed  in  street  work.     Fig.  37  shows  the  form  generally 
used  for  clay  or  gravel  excavation. 


Fig.  36.     Grading  Pick 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 


The  eye  of  the  pick  is  formed  generally  of  wrought  iron,  while 
the  points  are  of  steel.  The  weight  of  picks  ranges  from  4  to  9 
pounds. 


Fig.  37.     Clay  Pick 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 


Grubbing  Tools.  In  handling  brush,  stumps,  etc.,  such  tools 
as  the  bush  hooks,  Fig.  38,  the  bush  mattock,  Fig.  39,  and  the  axe 
mattock,  Fig.  40,  are  com- 
monly used.  These  are 
cutting  as  well  as  grading 
tools. 

Shovels.  Shovels,  Fig. 
41,  are  made  in  twTo  forms, 
square  and  round  pointed, 
usually  of  pressed  steel. 

Plows.    Plows  are  em- 

,  ,  ...  Fig.  38.     Bush  Hooks 

ployed  extensively  in  grad- 


Fig.  39.     Bush  Mattock 


Fig.  40.     Axe  Mattock 


ing,  special  forms  being  manufactured  for  the  purpose.    They  are 
known  as  "grading  plows",  "road  plows",  "township  plows",  etc. 


62 


HIGHWAY  CONSTRUCTION 


They  vary  in  form  according  to  the  kind  of  work  they  are  intended 
for,  viz,  loosening  earth,  gravel,  hardpan,  and  some  of  the  softer  rocks. 

These  plows  are  of  great  strength ; 
selected  white  oak,  rock  elm,  wrought 
steel,  and  iron  generally  being  used  in 
their  construction.  The  cost  of  oper- 
ating plows  ranges  from  2  to  5  cents 
per  cubic  yard,  depending  upon  the 
compactness  of  the  soil.  The  quan- 
tity of  material  loosened  will  vary 
from  2  to  5  cubic  yards  per  hour. 

Grading  Plow.  Fig.  42  shows  the 
form  usually  adopted  for  loosening 
earth.  This  plow  does  not  turn  the 
soil,  but  cuts  a  furrow  about  10  inches 
wide  and  of  a  depth  adjustable  up  to 
11  inches. 

In  light  soil  the  plows  are  oper- 
ated by  2  or  4  horses;  in  heavy  soil 
as  many  as  8  are  employed.  Grad- 
ing plows  vary  in  weight  from  100 
to  325  pounds. 


Fig.  41. 


Round  Pointed  and  Square 
Shovels 


Courtesy  of  Acme  Road  Machinery 
Company,  Frankfort,  New  York 


Fig.  42.     Typical  Road  Plow 
Courtesy  of  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

Hardpan  Plow.  Fig.  43  illustrates  a  plow  specially  designed 
for  tearing  up  macadam,  gravel,  or  similar  material.  The  point  is 
a  straight  bar  of  cast  steel  drawn  down  to  a  point,  and  can  be 
repaired  easilyr 


HIGHWAY  CONSTRUCTION 


63 


Scrapers.     Scrapers  are  used  generally  to  move  the  material 
loosened  by  plowing;  they  are  made  of  either  iron  or  steel,  and  in  a 


Fig.  43.     Typical  Hardpan  or  Rooter  Plow 
Courtesy  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 


variety  of  forms,  and  are  known  by  various  names,  as  "drag",  "buck", 
"pole",  and  "wheeled".  The  drag  scrapers  are  employed  usually 
on  short  hauls,  the  wheeled 
ones  on  long  hauls. 

Drag  Scrapers.  Drag 
scrapers,  Fig.  44,  are  made 
in  three  sizes.  The  smallest, 
for  one  horse,  has  a  capacity 
of  3  cubic  feet;  the  others, 
for  two  horses,  have  a 
capacity  of  5  to  7J  cubic 
feet.  The  smallest  weighs 
about  90  pounds,  and  the 


Fig.  44.     Drag  Scraper 

Courtesy  Western  Wheeled  Scraper  Company,. 
Aurora,  Illinois 


Fig.  45.     Buck  Scraper 

Courtesy  Western  Wheeled  Scraper  Company, 

Aurora,  Illinois 


larger  ones  from  94  to  102  pounds. 

Buck  Scrapers.  Buck  scrapers, 
Fig.  45,  are  made  in  two  sizes — 
two-rhorse,  carrying  7J  cubic  feet; 
four-horse,  ,12  cubic  feet. 

Pole  Scrapers.  Pole  scrapers 
are  designed  for  use  in  making  and 
leveling  earth  roads  and  for  cutting 
and  cleaning  ditches;  they  are  well 
adapted  also  for  moving  earth  short 
distances  at  a  minimum  cost. 


64  •          HIGHWAY  CONSTRUCTION 

Wheeled  Scrapers.  Wheeled  scrapers,  Fig.  46,  consist  of  a  metal 
box,  usually  steel,  mounted  on  wheels,  and  furnished  with  levers  for 
raising,  lowering,  and  dumping.  They  are  operated  in  the  same 


Fig.  46.     Typical  Wheeled  Scraper 
Courtesy  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

manner  as  drag  scrapers,  except  that  all  the  movements  are  made 
by  means  of  the  levers,  and  without  stopping  the  team.  By  their 
use  the  excessive  resistance  to  traction  of  the  drag  scraper  is  avoided. 


Fig.  47.     Contractor's  Barrow  with  Pressed-Steel  Tray 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 

Various  sizes  are  made,  ranging  in  capacity  from  10  to  17  cubic  feet. 
In  weight  they  range  from  350  to  700  pounds. 

Wheelbarrows.    Wheelbarrows  sometimes  are  constructed  of 
wood  and  are  employed  most  commonly  for  earthwork.    Their 


HIGHWAY  CONSTRUCTION  65 

capacities  range  from  2  to  2|  cubic  feet.     Weight  is  about  50  pounds. 

The  barrow,  Fig.  47,  has  pressed-steel  tray,  oak  frame,  and 
steel  wheels,  and  will  be  found  more  durable  in  the  maintenance 
department  than  the  all-wood  barrow.  Capacity  is  from  3J  to  5 
cubic  feet,  dependent  on  size  of  tray. 

The  barrow,  Fig.  48,  is  constructed  with  tubular-iron  frames 
and  steel  tray,  and  is  adaptable  to  the  heaviest  work,  such  as  mov- 


Fig.  48.     All  Steel  and  Iron  Concrete  Barrow 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 

ing  heavy  broken  stone,  etc.,  or  it  may  be  employed  with  advantage 
in  the  cleaning  department.  Capacity  from  3  to  4  cubic  feet. 
Weight  from  70  to  82  pounds. 

The  maximum  distance  to  which  earth  can  be  moved  econom- 
ically in  barrows  is  about  200  feet.    The  wheeling  should  be  per- 


Fig.  49.     Typical  Dump  Carts  for  Hauling  Earth,  Etc. 
Courtesy  of  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

formed  upon  planks,  whose  steepest  inclination  should  not  exceed  1 
in  12.  The  force  required  to  move  a  barrow  on  a  plank  is  about  -%% 
part  of  the  weight;  on  hard  dry  earth,  about  y^  part  of  the  weight. 
The  time  occupied  in  loading  a  barrow  will  vary  with  the 
character  of  the  material  and  the  proportion  of  wheelers  to  shovel- 
ers.  Approximately,  a  shoveler  takes  about  as  long  to  fill  a  barrow 


66 


HIGHWAY  CONSTRUCTION 


with  earth  as  a  wheeler  takes  to  wheel  a  full  barrow  a  distance  of 
about  100  or  120  feet  on  a  horizontal  plank  and  return  with  the 
empty  barrow. 


Fig.  50.     Rear  View  of  Typical  Dump  Wagon  Showing  Bottom  Open 
Courtesy  of  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

Carts.  The  cart  usually  employed  for  hauling  earth,  etc.,  is 
shown  in  Fig.  49.  The  average  capacity  is  22  cubic  feet,  and  the 
average  weight  is  800  pounds.  These  carts  are  furnished  usually 


Fig.  51.     Twenty-Yard  Dump  Car 
Courtesy  ojf  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

with  broad  tires,  and  the  body  is  balanced  so  that  the  load  is  evenly 
divided  about  the  axle. 

The  time  required  to  load  a  cart  varies  with  the  material. 
One  shoveler  will  require  about  as  follows:  clay,  7  minutes;  loam, 
6  minutes;  sand,  5  minutes. 


HIGHWAY  CONSTRUCTION  67 

Dump  Wagons.  The  use  of  dump  wagons,  Fig.  50,  for  moving 
excavated  earth,  etc.,  and  for  transporting  materials  such  as  sand, 
gravel,  etc.,  materially  shortens  the  time  required  for  unloading  the 
ordinary  form  of  contractor's  wagon;  having  no  reach  or  pole  con- 
necting the  rear  axle  with  the  center  bearing  of  the  front  axle,  they 
may  be  cramped  short  and  the  load  deposited  just  where  required. 
They  are  operated  by  the  driver,  and  the  capacity  ranges  from  35 
to  45  cubic  feet. 


Fig.  52.     Typical  Grader 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 

Dump  Cars.  Dump  cars,  Fig.  51,  are  made  to  dump  in  several 
different  ways,  viz,  single  or  double  side,  single  or  double  end,  and 
rotary  or  universal  dumpers. 

Dump  cars  may  be  operated  singly  or  in  trains,  as  the  magni- 
tude of  the  work  may  demand.  They  may  be  moved  by  horses 
or  small  locomotives.  They  are  made  in  various  sizes,  depending 
upon  the  gage  of  the  track  on  which  they  are  run.  A  common 
gage  is  20  inches,  but  it  varies  from  that  up  to  the  standard  railroad 
gage  of  56J  inches. 

Mechanical  Graders.  Mechanical  graders  are  used  extensively 
in  the  making  and  maintaining  of  earth  roads.  They  excavate 
and  move  earth  more  expeditiously  and  economically  than  can  be 


HIGHWAY  CONSTRUCTION  69 

done  by  hand;  they  are  called  by  various  names,  such  as  "road 
machines",  "graders",  "road  hones",  etc. 

Simple  Graders.  Briefly  described,  graders  consist  of  a  large 
blade,  Fig.  52,  made  entirely  of  steel,  iron,  or  wood  shod  with  steel, 
which  is  so  arranged  by  a  mechanism  attached  to  the  frame  from 
which  it  is  suspended  that  it  can  be  adjusted  and  fixed  in  any 
direction  by  the  operator.  In  their  action  they  combine  the  work 
of  excavating  and  transporting  the  earth.  They  have  been 
employed  chiefly  in  the  forming  and  maintenance  of  earth  roads, 
but  also  may  be  used  advantageously  in  preparing  the  subgrade 
surface  of  roads  for  the  reception  of  broken  stone  or  other 
improved  covering. 

Elevating  Graders.  Some  graders  combine  the  function  of 
elevating  the  material,  of  excavating  it  from  side  ditches,  and 
of  loading  it  automatically  into  carts  or  wagons.  Briefly  described, 
the  machine,  Fig.  53,  consists  of  a  plow  which  loosens  and  raises 
the  earth,  depositing  it  upon  a  transverse  carrying  belt,  which  con- 
veys it  from  excavation  to  embankment.  Carrier  frames  of  two 
or  three  different  lengths  are  provided  with  the  machine,  the  distance 
of  the  end  of  the  elevator  from  the  plow  varying  from  15  to  30  feet. 
The  carrier  belt  is  of  heavy  3-ply  rubber  3  feet  wide. 

The  plow  and  carrier  are  supported  by  a  strong  trussed  frame- 
work resting  on  heavy  steel  axles  and  broad  wheels.  The  large 
rear  wheels  are  ratcheted  upon  the  axle,  and  connected  with  strong 
gearing  which  propels  the  carrying  belt  at  right  angles  to  the  direc- 
tion in  which  the  machine  is  moving. 

The  wheels  and  trusses  are  low  and  broad,  occupying  a  space 
8  feet  wide  and  14  feet  long,  exclusive  of  the  side  carrier.  This 
enables  it  to  work  on  hillsides  where  any  wheeled  implements  can 
be  used.  Notwithstanding  its  large  size  it  is  so  flexible  that  it  may 
be  turned  around  on  a  16-foot  embankment.  Pilot  wheels  and 
levers  enable  the  operator  to  raise  or  lower  the  plow  or  carrier  at 
pleasure. 

For  motive  power,  12  horses — 8  driven  in  front,  4  abreast,  and 
4  in  the  rear  on  a  push  cart — are  usually  employed. 

When  the  teams  are  started,  the  operator  lowers  the  plow  and 
throws  the  belting  into  gear,  and  as  the  plow  raises  and  turns  the 
earth  to  the  side  the  belt  receives  and  delivers  it  at  the  distance  for 


70 


HIGHWAY  CONSTRUCTION 


which  the  carrier  is  adjusted,  forming  either  excavation  or  embank- 
ment, as  the  case  may  be. 

When  it  becomes  necessary  to  deliver  the  excavated  earth 
beyond  the  capacity  of  the  machine,  the  earth  is  loaded  upon  wagons, 


Fig.  54.     Two  Views  of  Elevating  Graders  Loading  Earth  into  Dump  Wagons 
Courtesy  of  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

then  conveyed  to  any  distance.  By  adjusting  the  height  of  the 
carrier,  the  wagons  are  driven  under  it,  Fig.  54,  and  loaded  with 
H  to  1J  yards  of  earth  in  from  20  to  30  seconds.  When  one  wagon 
turns  out  with  its  load,  another  drives  under  the  carrier,  and  the 


HIGHWAY  CONSTRUCTION  71 

machine  thus  loads  600  to  800  wagons  per  day.  It  is  claimed  that 
with  six  teams  and  three  men  it  is  capable  of  excavating  and  placing 
in  embankment  from  1,000  to  1,500  cubic  yards  of  earth  in  10  hours, 
or  of  loading  from  600  to  800  wagons  in  the  same  time,  and  that  the 
cost  of  this  handling  is  from  1J  to  2J  cents  per  cubic  yard. 

Points  to  be  Considered  in  Selecting  a  Road  Machine.  In  the 
selection  of  a  road  machine  the  following  points  should  be  carefully 
considered :  thoroughness  and  simplicity  of  its  mechanical  construc- 
tion; material  and  workmanship  used  in  its  construction;  safety 
to  the  operator;  ease  of  operation;  lightness  of  draft;  and  adapta- 
bility to  general  road  work,  ditching,  etc. 

Care  of  Road  Machines.  The  road  machine  when  not  in  use 
should  be  stored  in  a  dry  house  and  thoroughly  cleaned,  its  blade 
brushed  clean  from  all  accumulations  of  mud,  wiped  thoroughly 
dry,  and  well  covered  with  grease  or  crude  oil.  The  axles,  journals, 
and  wearing  parts  should  be  kept  well  oiled  when  in  use,  and  an 
extra  blade  should  be  kept  on  hand  to  avoid  stopping  the  machine 
while  the  dulled  one  is  being  sharpened. 

Surface  Graders.  The  surface  grader  is  used  for  removing 
earth  previously  loosened  by  a  plow.  It  is  operated  by  one  horse. 


Fig.  55.     Simple  Road  Leveler 

The  load  may  be  retained  and  carried  a  considerable  distance,  or 
it  may  be  spread  gradually  as  the  operator  desires.  It  is  also 
employed  to  level  off  and  trim  the  surface  following  the  scrapers. 

The  blade  is  of  steel,  \  inch  thick,  15  inches  wide,  and  30  inches 
long.  The  beam  and  other  parts  are  of  oak  and  iron.  Weight 
about  60  pounds. 

Road  Leveler.  The  road  leveler,  Fig.  55,  is  used  for  trimming 
and  smoothing  the  surface  of  earth  roads.  It  is  largely  employed  in 
the  spring  when  the  frost  leaves  the  ground. 


72 


HIGHWAY  CONSTRUCTION 


The  blade  is  of  steel,  J-inch  thick  by  4  inches  by  72  inches,  and 
is  provided  with  a  seat  for  the  driver.  It  is  operated  by  a  team  of 
horses.  Weight  about  150  pounds. 

Ditching  Tools.  The  tools  employed  for  digging  the  ditches 
and  shaping  the  bottom  to  fit  the  drain  tiles  are  shown  in  Fig.  56. 
They  are  convenient  to  use,  and  expedite  the  work  by  avoiding 
unnecessary  excavation. 

The  tools  are  used  as  follows:  Nos.  3,  4,  and  5  are  used  for 
digging  the  ditches;  Nos.  6  and  7  for  cleaning  and  rounding  the 


Fig.  56.     Typical  Tools  Used  for  Digging  Ditches 

bottom  of  the  ditch  for  round  tile;  No.  2  is  used  for  shoveling  out 
loose  earth  and  leveling  the  bottom  of  the  ditch;  No.  1  is  used  for 
the  same  purpose  when  the  ditch  is  intended  for  "sole"  tile. 

Sprinkling  Wagons.  A  convenient  form  of  sprinkling  wagon 
for  suburban  streets  and  country  roads  is  shown  in  Fig.  57.  The 
tank  is  of  12  gage  steel  and  its  capacity  is  380  to  600  gallons. 


HIGHWAY  CONSTRUCTION 


73 


Road  Rollers.     Horse-Drawn  Rollers.    There  are  a  number  of 
types  of  horse-drawn  rollers  on  the  market,  consisting  essentially 


Fig.  57.     Steel  'lank  Sprinkling  Wagon 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 

of  a  hollow  cast-iron  cylinder   4  to  5  feet  long,  5  to  6  feet  in 
diameter,    and   weighing   from   3    to    6   tons.    Some    forms    are 


Fig.  58.     Ten-Ton  Steam-Driven  Road  Roller 
Courtesy  of  Charles  Longenecker  &  Company,  New  York  City 

provided  with  boxes  in  which  stone  or  iron  may  be  placed  to 
increase  the  weight,  and  some  have  closed  ends  and  may  be  filled 


74  HIGHWAY  CONSTRUCTION 

with  water  or  sand.  The  use  today  of  small  gasoline  road  rollers 
makes  this  type  less  prevalent  than  formerly. 

Power-Propelled  Rollers.  The  rollers  employed  for  compacting 
the  natural  soil  and  all  forms  of  broken-stone  pavements  usually 
are  of  the  three-wheel  type,  operated  by  steam  or  gasoline,  Fig.  58. 
They  generally  are  arranged  to  move  at  two  speeds,  low  and  high; 
the  low  speed  is  from  2  to  3  miles  per  hour  and  the  high  speed  from 
4  to  5  miles.  The  low  speed  is  employed  for  compacting  the  natural 
soil  and  the  foundation;  the  high  speed  is  employed  for  finishing 
the  surface.  The  driving  wheels  are  furnished  with  lock  pins  or 
differential  gears  to  permit  them  to  accommodate  themselves  auto- 
matically to  the  difference  in  speeds  when  operating  on  sharp  curves. 
They  vary  in  weight  from  10  to  20  tons. 

Scarifiers.  The  implement  used  for  breaking  up  a  broken- 
stone  road  preparatory  to  applying  a  new  surface  is  called  a  "scari- 


ng. 59.     Scarifier,  for  Quick  and  Economical  Repair  of  Macadam  Roads 
Courtesy  of  Charles  Longenecker  &  Company,  New  York  City 

fier",  Fig.  59.  It  usually  consists  of  a  cast-iron  block,  weighing 
about  3  tons,  mounted  on  2  or  4  wheels;  the  block  is  fitted  with  a 
series  of  spikes  or  picks,  arranged  either  in  one  line,  or  in  two  lines 
forming  a  V;  means  are  provided  for  adjusting  the  depth  to  which 
the  picks  penetrate,  the  maximum  depth  being  about  6  inches. 
The  scarifier  is  operated  by  being  attached  to  the  rear  of  a  steam 
roller  or  traction  engine  which  hauls  it  over  the  road. 

NATURAL=SOIL  ROADS 

Earth   Roads.    The  term   "earth  road"   is  applied  to  roads 
where  the  surface  consists  of  the  native  soil;  this  class  of  road  is  the 


HIGHWAY  CONSTRUCTION  75 

most  common  and  cheapest  in  first  cost.  At  certain  seasons  of  the 
year  earth  roads,  when  properly  cared  for,  are  second  to  none,  but 
during  the  spring  and  wet  seasons  they"  are  very  deficient  in  the 
important  requisite  of  hardness,  and  are  almost  impassable. 

For  the  construction  of  new  earth  roads,  all  the  principles  pre- 
viously discussed  relating  to  alignment,  grades,  drainage,  width,  etc., 
should  be  followed  carefully.  The  crown  or  transverse  contour 
should  be  greater  than  in  stone  roads;  12  inches  at  the  center  in  25 
feet  will  be  sufficient. 

Drainage  is  especially  important,  because  the  material  of  the 
road  is  more  susceptible  to  the  action  of  water,  and  more  easily 
destroyed  by  it  than  are  the  materials  used  in  the  construction  of  the 
better  class  of  roads.  When  water  is  allowed  to  stand  upon  the 
road,  the  earth  is  softened,  the  wagon  wheels  penetrate  it,  and  the 
horses'  feet  mix  and  knead  it  until  it  becomes  impassable  mud.  The 
action  of  frost  is  also  apt  to  be  disastrous  upon  the  more  permeable 
surface  of  the  earth  road,  having  the  effect  of  swelling  and  heaving 
the  roadway  and  throwing  its  surface  out  of  shape.  It  may  in  fact 
be  said  that  the  whole  problem  of  the  improvement  and  mainte- 
nance of  ordinary  country  roads  is  one  of  drainage. 

In  the  preparation  of  the  wheelway  all  stumps,  brush,  vegetable 
matter,  rocks,  and  boulders  should  be  removed  from  the  surface  and 
the  resulting  holes  filled  in  with  clean  earth.  The  roadbed,  having 
been  brought  to  the  required  grade  and  crown,  should  be  thoroughly 
rolled;  all  inequalities  appearing  during  the  rolling  should  be  filled 
up  and  re-rolled. 

Care  of  Earth  Roads.  If  the  surface  of  the  roadway  is  prop- 
erly formed  and  kept  smooth,  the  water  will  be  shed  into  the  side 
ditches  and  do  comparatively  little  harm;  but  if  it  remains  upon  the 
surface,  it  will  be  absorbed  and  convert  the  road  into  mud.  All 
ruts  and  depressions  should  be  filled  up  as  soon  as  they  appear. 
Repairs  should  be  attended  to  particularly  in  the  spring.  At  that 
season  the  judicious  use  of  a  road  machine  and  rollers  will  make  a 
smooth  road.  In  summer  when  the  surface  gets  rough  it  can  be 
improved  by  running  a  harrow  over  it;  if  the  surface  is  a  little  muddy 
this  treatment  will  hasten  the  drying. 

During  the  fall  the  surface  should  be  repaired,  with  special 
reference  to  putting  it  in  shape  to  withstand  the  ravages  of  winter. 


76  HIGHWAY  CONSTRUCTION 

Saucer-like  depressions  and  ruts  should  be  filled  up  with  clean  earth 
similar  to  that  of  the  roadbed  and  tamped  into  place. 

The  side  ditches  should  be  examined  in  the  fall  to  see  that  they 
are  free  from  dead  weeds  and  grass,  and  late  in  winter  they  should 
be  examined  again  to  see  that  they  are  not  clogged.  The  mouths 
of  culverts  should  be  cleaned  of  rubbish  and  the  outlet  of  tile  drains 
opened.  Attention  to  the  side  ditches  will  prevent  overflow  and 
washing  of  the  roadway,  and  also  will  prevent  the  formation  of 
ponds  at  the  roadside  and  the  consequent  saturation  of  the  roadbed. 

Holes  and  ruts  should  not  be  filled  with  stone,  bricks,  gravel, 
or  other  material  harder  than  the  earth  of  the  roadway  as  the  hard 
material  will  not  wear  uniformly  with  the  rest  of  the  road,  but 
produce  bumps  and  ridges,  and  usually  result  in  making  two  holes, 
each  larger  than  the  original  one.  It  is  bad  practice  to  cut  a  gutter 


Fig.  60      fcsteel  Road  Drag 
Courtesy  of  Western  Wheeled  Scraper  Company,  Aurora,  Illinois 

from  a  hole  to  drain  it  to  the  side  of  the  road.  Filling  is  the  proper 
course,  whether  the  hole  is  dry  or  contains  mud. 

The  maintaining  of  smooth  surfaces  on  all  classes  of  earth  roads 
will  be  assisted  and  cheapened  greatly  by  the  frequent  use  of  a 
roller  (either  steam  or  horse)  and  any  one  of  the  various  forms  of 
road  grading  and  scraping  machines.  In  repairing  an  earth  road 
the  plow  should  not  be  used.  It  breaks  up  the  surface  which  has 
been  compacted  by  time  and  travel. 

In  the  maintenance  of  earth  roads  the  road  drag,  Fig.  60,  or 
some  similar  device,  is  indispensable.  The  drag  should  be  light 
and  should  be  hauled  along  the  road  at  an  angle  of  about  45  degrees, 
so  that  only  a  small  amount  of  earth  is  pushed  to  the  center  of  the 


HIGHWAY  CONSTRUCTION  77 

road.  The  driver  should  ride  on  the  drag  and  not  drive  faster 
than  a  walk.  Dragging  should  begin  on  the  side  of  the  road,  or 
wheel  track,  and  return  on  the  opposite  side.  Unless  the  road  is 
in  good  condition,  it  should  be  dragged  after  every  heavy  rain. 

In  the  maintenance  of  clay  roads  neither  sods  nor  turf  should 
be  used  to  fill  holes  or  ruts;  for,  though  at  first  deceptively  tough, 
they  soon  decay  and  form  the  softest  mud.  Neither  should  the  ruts 
be  filled  with  field  stones;  they  will  not  wear  uniformly  with  the 
rest  of  the  road,  but  will  produce  hard  ridges. 

Trees  and  close  hedges  should  not  be  allowed  within  200  feet 
of  a  clay  road.  It  requires  all  the  sun  and  wind  possible  to  keep  its 
surface  in  a  dry  and  hard  condition. 

Sand  Roads.  The  aim  in  the  improvement  of  sand  roads  is  to 
have  the  wheelway  as  narrow  and  well  defined  as  possible,  so  as  to 
have  all  vehicles  run  in  the  same  track.  An  abundant  growth 
of  vegetation  should  be  encouraged  on  each  side  of  the  wheelway, 
for  by  this  means  the  shearing  of  the  sand  is,  in  a  great  measure, 
avoided.  Ditching  beyond  a  slight  depth  to  carry  away  the  rain 
water  is  not  desirable,  for  it  tends  to  hasten  the  drying  of  the  sands, 
which  is  to  be  avoided.  Where  possible  the  roads  should  be  over- 
hung with  trees,  the  leaves  and  twigs  of  which,  catching  on  the 
wheelway,  will  serve  still  further  to  diminish  the  effect  of  the  wheels 
in  moving  the  sands  about.  If  clay  can  be  obtained,  a  coating  6 
inches  thick  will  be  found  a  most  efficient  and  economical  improve- 
ment. A  coating  of  4  inches  of  loose  straw  will,  after  a  few  days' 
travel,  grind  into  the  sand  and  become  as  hard  and  firm  as  a  dry 
clay  road. 

Sand=Clay  Roads.  A  sand-clay  road  is  formed  by  mixing 
clay  and  sand  in  such  proportions  that  the  clay  will  just  fill  the 
voids  in  the  sand,  and  produce  a  mixture  that  is  neither  sticky  nor 
friable,  but  coheres  in  a  comparatively  dry  plastic  mass  when  com- 
pacted with  pressure.  If  an  insufficient  amount  of  clay  is  used, 
the  mixture  will  not  bind;  if  an  excess  of  clay  is  used,  the  road  will 
be  sticky  and  muddy  after  a  fall  of  rain. 

The  grains  of  sand  furnish  the  hard  material  to  resist  the 
abrasion  of  the  traffic;  the  clay  provides  the  cementing  or  binding 
medium  to  hold  the  sand  together.  All  clays  are  not  equally  satis- 
factory as  binders,  owing  to  the  diversity  of  their  origin.  A  common 


78  HIGHWAY  CONSTRUCTION 

test  for  clay  suitable  for  road  purposes  is  to  apply  a  wet  finger 
to  a  piece  of  clay;  if  the  clay  adheres  to  the  finger,  it  may  be  assumed 
reasonably  that  it  will  adhere  to  the  sand. 

The  natural  sand  soils  and  the  naturel  clay  soils  are  improved 
by  the  application  of  the  sand-clay  mixture,  the  method  of  applying 
it  being  varied  according  to  which  kind  of  soil  is  to  be  treated. 

Applying  Sand-Clay  Mixture  to  Clay  Soil.  In  the  treatment 
of  a  clay  soil,  the  soil  is  plowed  to  a  depth  of  6  to  8  inches;  then 
pulverized  by  harrowing,  and,  if  necessary,  by  rolling  with  a  light 
roller  and  again  harrowing.  After  the  clay  is  thoroughly  pulverized, 
the  sand  is  spread  over  the  surface  in  a  layer  from  6  to  8  inches 
thick,  and  the  sand  and  clay  are  thoroughly  mixed  by  continued 
harrowing.  After  the  dry  mixing  has  been  accomplished  satis- 
factorily, the  surface  is  moistened  slightly  by  sprinkling  with  water, 
then  compacted  by  rolling,  after  which  a  road  machine  or  grader 
is  used  to  give  the  required  crown;  and  then  the  roller  is  again  applied 
until  the  surface  becomes  smooth  and  hard. 

Applying  Sand-Clay  Mixture  to  Sand  Soil.  In  the  treatment 
of  a  sand  soil,  the  clay  is  spread  over  the  surface  in  a  layer,  ranging 
from  4  to  8  inches  thick;  then  mixed  with  the  sand  by  harrowing. 
After  that  it  is  sprinkled  heavily  with  water  and  again  worked 
with  the  harrow;  then  it  is  shaped  and  rolled  in  the  same  manner 
as  stated  above  for  a  clay  soil. 

The  sand-clay  roads  require  considerable  attention,  after 
completion,  to  eliminate  weak  or  defective  spots  by  applying  sand 
or  clay,  as  may  be  required. 

Application  of  Oil  to  Sand  and  Gravel  Soils.  Sand  and  gravel 
soils  are  improved  by  the  application  of  crude  petroleum  or  asphal- 
tic  oils.  The  oil  abates  dust;  forms  a  non-absorbent  surface  which 
turns  off  rain  water  and  decreases  the  amount  of  mud;  and  furnishes 
a  dark-colored  road  surface  which  is  more  pleasing  to  the  eye  than 
the  ordinary  light  color. 

The  roadbed  is  prepared  to  receive  the  oil  by  grading,  shaping, 
and  rolling.  The  oil  is  applied  to  the  prepared  surface  by  sprinkling 
from  tank  wagons;  the  oil  coat  is  covered  with  a  thin  layer  of  sand, 
after  which  the  roller  is  applied  again.  If  during  the  rolling  the 
surface  becomes  sticky,  or  dry  and  dusty,  dry  sand  or  more  oil  is 
added  as  required. 


HIGHWAY  CONSTRUCTION  79 

ROADS  WITH  SPECIAL  COVERINGS 

Elements  of  a  Road  Covering.  The  wheelways  of  roads  and 
streets  are  prepared  for  traffic  by  placing  upon  the  natural  soil  a 
covering  of  some  suitable  material  which  will  furnish  a  comparatively 
smooth  surface  on  which  the  resistance  to  traction  will  be  reduced 
to  the  least  possible  amount,  and  over  which  all  classes  of  vehicles 
may  pass  with  safety  and  expedition  at  all  seasons  of  the  year. 
The  covering  usually  consists  of  two  parts:  a  foundation,  and  a 
wearing  surface. 

The  functions  of  the  foundation  are  as  follows:  (1)  to  protect 
the  soil  from  disturbance  and  the  injurious  effects  of  surface  water; 

(2)  to  transmit  to  and  distribute  over  a  sufficiently  large  area  of 
the  soil  the  weight  of  the  loads  imposed  upon  the  wearing  coat; 

(3)  to  support  unyieldingly  the  wearing  surface  and  the  loads  coming 
upon  it. 

The  efficiency  of  the  wearing  surface  depends  entirely  upon 
the  quality  of  the  foundation.  If  the  foundation  be  weak,  the 
wearing  surface  will  be  disrupted  speedily,  no  matter  how  well 
constructed. 

FOUNDATIONS 

Materials.  The  foundation,  when  once  constructed,  should 
not  require  to  be  disturbed  nor  reconstructed.  The  materials 
employed  in  its  construction  may  be  the  cheapest  available,  such 
as  local  rock,  gravel,  sand,  furnace  slag,  etc.,  the  important  point 
in  the  design  being  to  provide  sufficient  thickness,  so  that  when 
consolidated  it  will  maintain  its  form  under  the  heaviest  traffic 
liable  to  come  upon  it.  If  the  foundation  and  the  covering  yield 
under  the  load,  an  upheaval  is  caused  that  disrupts  the  bond  and 
hastens  the  destruction  of  the  road. 

Thickness.  The  thickness  of  the  foundation  depends  upon 
the  supporting  power  of  the  natural  soil  and  the  weight  of  the  loads 
coming  upon  the  wearing  surface.  The  supporting  or  bearing 
power  of  the  soil  can  be  ascertained  by  direct  test,  and  the  weight 
of  the  loads  by  a  survey  of  the  traffic  plus  a  provision  for  future 
increase. 

Recent  tests  indicate  that  non-porous  soils  from  which  the 
subsoil  water  is  removed  by  drainage  will  support  in  their  worst 


80  HIGHWAY  CONSTRUCTION 

condition  a  load  of  about  4  pounds  per  square  inch;  and  that  if 
the  thickness  of  the  foundation  be  adjusted  to  the  traffic  on  this 
basis  it  will  be  safe  at  all  seasons  of  the  year. 

Methods  of  Calculating  Thickness  of  Covering.  There  are  two 
theories  as  to  the  manner  in  which  pressure  of  a  loaded  wheel  is 
transmitted  from  the  surface  of  the  covering  to  the  natural  soil: 
(1)  that  the  pressure  on  the  soil  varies  inversely  as  the  cube  of  the 
thickness  of  the  foundation  and  the  wearing  surface;  (2)  that  the 
pressure  is  transmitted  downwards  in  the  form  of  a  truncated  cone, 
the  lines  of  which  diverge  at  an  angle  varying  from  30  to  50  degrees 
from  the  vertical,  according  to  the  solidity  of  the  covering.  If 
the  surface  of  the  road  is  uneven  or  obstructed  by  loose  stones, 
the  lines  of  pressure  are  more  concentrated  when  the  wheels  pass 
over  such  obstacles. 

The  latter  theory  is  the  one  most  frequently  applied.  The 
calculation  is  performed  as  follows:  Let  P  be  distributed  pressure 
on  the  soil,  per  square  inch;  A,  length  of  arc  of  wheel  tire  in  contact 
with  surface  in  square  inches;  W,  width  of  tire  in  inches;  L,  load 
carried  by  wheel  in  pounds;  F,  depth  of  wearing  surface  and  foun- 
dation in  inches;  C,  area  of  contact  equal  to  AxW;  and  B,  area 
of  base  at  surface  of  natural  soil.  The  area  of  the  base  is 

B  =  (2F+A)   &F+W) 
The   distributed   pressure   is 

p=     _  L_  =  L 

(2F+A)  (2F+W)     B 

Assuming  that  the  load  is  1,000  pounds  per  inch  of  tire  width;  the 
tire,  3  inches;  length  of  contact  3  inches;  total  thickness  of  the 
wearing  coat  and  the  foundation  12  inches;  the  pressure  on  the 
soil  is 


1000X3  _       3000   =          =  llolb  j 

"(2X12+3)  (2X12+3)"  27X27"   729  ~ 

According  to  this  theory  the  thickness  of  the  covering  varies 
from  4  to  16  inches,  the  smallest  thickness  being  placed  upon  gravel 
or  sand  and  the  greatest  upon  clay. 

Preparation  of  Foundation.  The  preparation  of  the  foundation 
involves  two  distinct  operations:  (1)  preparation  of  the  natural 
soil;  and  (2)  placing  an  artificial  foundation  upon  the  Drepared 
natural  soil. 


HIGHWAY  CONSTRUCTION  81 

The  essentials  necessary  to  the  preparation  of  the  natural 
soil  are:  (1)  the  entire  removal  of  perishable  vegetable  and  yielding 
matter;  (2)  the  drainage  of  the  soil  where  necessary;  (3)  the  improv- 
ing of  the  bearing  power  of  the  soil  where  required;  and  (4)  compact- 
ing the  soil. 

All  soils  are  improved  by  rolling,  and  weak  spots,  which  other- 
wise would  pass  unnoticed,  are  discovered.  However,  care  must 
be  taken  that  the  weight  of  the  roller  employed  is  not  too  great  for 
the  bearing  power  of  the  soil ;  if  it  exceed  this  the  surface  of  the  soil 
will  be  formed  into  a  series  of  undulations  that  will  cause  the  wear- 
ing coat  to  fail;  the  same  condition  may  be  produced  by  excessive 
rolling  with  a  comparatively  light  roller.  Each  soil  requires  different 
treatment. 

Soils  of  a  siliceous  and  calcareous  nature  may  be  improved  by 
drainage  and  the  addition  of  a  layer  of  clay  2  to  6  inches  thick, 
mixed  with  the  soil  and  compacted  by  rolling.  The  argillaceous 
and  allied  soils,  owing  to  their  retentive  nature,  are  very  unstable 
under  the  action  of  water  and  frost,  and  in  their  natural  condition 
afford  a  defective  foundation.  They  are  improved  by  thorough 
drainage  and  the  admixture  of  sand  well  rolled,  together  with  the 
placing  upon  the  surface  of  the  compacted  soil  a  layer  2  to  6  inches 
thick  of  sand,  slag,  cinders,  or  other  material  of  a  similar  nature, 
and  then  compacting  it  by  sprinkling  with  water  and  rolling. 

Types  of  Foundation  to  Be  Used.  The  essential  requisite  in  the 
construction  of  the  artificial  foundation  is  that  it  be  a  dense  mass, 
and  the  type  of  foundation  to  be  employed  varies  with  the  char- 
acter of  the  wearing  surface.  For  the  various  types  of  broken- 
stone  surfaces,  the  foundation  may  be  composed  of  blocks  of  stone 
(ledge  rock  or  fieldstones),  roughly  shaped  to  a  rectangular  form, 
ranging  in  width  and  depth  from  6  to  8  inches  and  in  length  from 
6  to  16  inches.  They  are  set  by  hand  on  the  soil  bed  with  the  length 
at  right  angles  to  the  axis  of  the  roadway,  so  arranged  that  they 
break  joints.  The  edges  that  project  above  the  subgrade  level  are 
broken  off  with  hand  hammers,  and  the  spaces  between  them  are 
filled  with  chips  of  stone  well  packed  and  wedged  in.  The  blocks 
are  brought  then  to  a  firm  bearing  by  rolling  with  a  steam  roller, 
after  which  the  wearing  surface  is  laid.  The  foundation  also  may 
be  composed  of  broken  stone,  gravel,  or  furnace  slag  so  graded  that 


82  HIGHWAY  CONSTRUCTION 

the  voids  will  be  reduced  to  the  smallest  possible  amount.  The 
voids  may  be  filled  with  stone  dust;  a  mixture  of  sand  and  clay;  a 
mortar  and  grout  composed  of  hydraulic  cement  and  sand;  bitu- 
minous cement;  or  hydraulic-cement  concrete,  mixed  and  placed 
upon  the  soil  bed. 

WEARING  SURFACES 

Functions  of  Wearing  Surface.  The  office  of  the  wearing  sur- 
face is  to  protect  the  foundation  from  the  wear  of  the  traffic  and  the 
effects  of  surface  water,  and  to  support  the  weight  of  the  traffic  and 
transmit  it  to  the  foundation.  To  render  efficient  service  to  the 
traffic,  it  must  furnish  a  comparatively  smooth  unyielding  surface 
that  affords  good  foothold  for  draft  animals  and  good  adhesion  for 
motor  vehicles,  and  on  which  the  resistance  to  traction  will  be  a 
minimum.  To  fill  its  office  satisfactorily  the  material  of  which  it  is 
composed  must  possess  strength  to  resist  crushing  and  abrasion, 
and  its  fabric  must  be  practically  impervious.  To  render  economical 
service,  it  must  possess  the  power  of  resisting  the  action  of  the 
destroying  agencies  for  a  reasonable  length  of  time  before  it  becomes 
unfit  for  use.  For  this  purpose  it  must  possess  the  resisting  quali- 
ties previously  stated,  and  it  must  also  possess  a  certain  thickness; 
this  thickness  will  depend  upon  the  character  of  the  material 
employed  and  its  rate  of  wear  under  the  given  traffic  and  atmospheric 
conditions.  Economy  is  not  promoted  by  using  a  thick  wearing 
surface,  as  under  heavy  traffic  it  will  be  so  worn  in  a  few  years  as  to 
be  unserviceable,  and  under  light  traffic  it  will  be  decomposed  before 
it  is  worn  out.  In  either  case  it  must  be  removed  and  the  portion 
so  removed  is  waste;  therefore,  only  such  thickness  as  will  give 
efficient  service  during  a  few  years  should  be  adopted. 

Thickness.  The  measure  for  the  economical  thickness  of  any 
type  of  wearing  surface  is  that  the  annual  interest  charge  on  the 
first  cost  plus  the  annual  depreciation  shall  be  a  minimum.  To 
apply  this  measure  it  is  necessary  to  know  the  amount  of  traffic  and 
the  loss  of  thickness  due  to  wear. 

Classification  of  Wearing  Surfaces.  The  wearing  surfaces 
most  commonly  employed  for  roads  and  streets  are  composed  of: 
(1)  gravel,  broken  stone,  furnace  slag,  and  similar  granular  materials 
bound  with  colloidal  cement  formed  by  the  action  of  water  on  the 
plastic  elements  of  rock  and  clay;  (2)  broken  stone,  gravel,  and  sand 


HIGHWAY  CONSTRUCTION  83 

bound  with:  (a)  bituminous  cement;  (b)  hydraulic  cement;  (3)  stone 
blocks;  (4)  brick;  (5)  wood  blocks. 

In  type  (1),  a  certain  amount  of  moisture  is  essential  to  success- 
ful binding.  When  this  is  lacking,  as  in  the  summer  season,  the 
binding  material  becomes  dry  and  brittle,  and  the  fragments  at  the 
surface  are  displaced  by  the  action  of  the  traffic;  an  excess  of  mois- 
ture destroys  the  binding  power;  and  the  surface  is  quickly  broken 
up  by  the  traffic. 

Wearing  surfaces  of  type  (2a)  are  usually  limited  in  life  not 
merely  by  the  wear  of  traffic,  but  by  the  fact  that  all  bitumens. slowly 
alter  in  chemical  composition  when  exposed  to  atmospheric  action, 
and  in  time  become  brittle.  Type  (2b)  is  subject  to  cracking  under 
expansion  and  contracting,  due  to  changes  of  temperature,  and  is 
liable  to  wrear  unevenly  owing  to  irregularity  in  mixing  and  the  segre- 
gation of  the  ingredients  while  the  concrete  is  being  put  in  place. 
When  a  defective  spot  begins  to  wear,  it  extends  very  rapidly  under 
the  abrasive  action  of  the  traffic. 

The  materials  of  types  (3)  and  (4)  seldom  rot  or  disintegrate 
and,  when  the  pavement  is  well  constructed,  are  eminently  enduring 
and  generally  render  satisfactory  service.  Since  the  use  of  creosote 
and  other  preservatives  has  increased  the  service  life  of  wood  blocks, 
type  (5),  by  lessening  their  tendency  to  decay,  they  have  come  into 
extensive  use  for  street  paving. 

Gravel  Roads 

Gravel.  Gravel  consists  of  smooth  and  somewhat  rounded 
stones,  varying  in  size  from  small  grains  to  pebbles  4  or  more  inches 
in  diameter.  It  is  found  mixed  with  sand,  on  the  banks  and  in  the 
beds  of  rivers;  and  in  deposits  on  the  land,  mixed  with  clay  and  other 
mineral  substances,  such  as  limestone  and  oxide  of  iron,  from  which 
it  derives  a  distinctive  name.  Gravel  of  the  latter  class  is  called 
cementatious  and  when  suitably  prepared  cements  together,  forming 
a  very  satisfactory  roadway  for  light  traffic,  producing  but  little 
dust  in  dry  weather  and  costing  little  to  maintain. 

Preparation  of  Gravel.  Gravel  is  best  prepared  for  use  by 
screening  into  three  grades:  grade  (I),  containing  the  stones  retained 
by  a  IJ-inch  mesh  screen  and  passing  a  2J-inch  mesh;  grade  (2), 
containing  the  stones  retained  by  a  i-inch  mesh  and  passing  a  1  J-inch 


84  HIGHWAY  CONSTRUCTION 

mesh;  grade  (3),  containing  all  the  material  passing  the  J-inch 
mesh.  The  voids  in  grade  (1)  are  determined,  and  enough  of  grade 
(3)  added  slightly  more  than  to  fill  them;  the  two  are  intimately 
and  evenly  mixed  and  the  mixture  is  used  for  the  first  or  lower 
course.  The  voids  in  grade  (2)  are  determined  and  a  sufficient 
quantity  of  grade  (3)  added  to  fill  them ;  the  two  are  mixed  and  used 
for  the  top  course.  The  mixture  should  be  combined  very  evenly 
so  that  the  fine  material  is  mixed  uniformly  with  the  coarse ;  and  in 
spreading  the  mixture,  care  should  be  taken  to  avoid  separating  it 
or  allowing  the  fine  material  to  settle  to  the  bottom. 

If  the  gravel  is  deficient  in  binding  material,  the  latter  may  be 
added  in  the  form  of  clay,  loam,  limestone  screenings,  shale,  or  marl, 
the  amount  added  ranging  from  10  to  15  per  cent.  An  excess  (20 
per  cent)  of  clay  causes  the  gravel  to  pack  quickly  and  to  present  a 
good  appearance  under  the  rolling;  but  in  dry  weather  the  road  will 
ravel,  become  defective  and  dusty,  and  in  wet  weather  it  will  be 
muddy.  Clean  smooth  gravel  will  not  consolidate  without  a  binder 
and,  unless  this  is  of  very  good  quality,  a  road  made  with  it  will 
prove  unsatisfactory. 

Laying  the  Gravel.  On  the  natural-soil  bed  properly  graded 
and  compacted,  the  prepared  gravel  is  spread  uniformly  to  the 
depth  desired — usually  6  inches.  Then  it  is  compacted  by  rolling 
with  a  steam  roller,  after  which  it  is  moistened  by  sprinkling  with 
water,  and  the  rolling  is  repeated.  The  sprinkling  and  rolling  are 
repeated  as  often  as  may  be  required,  until  the  stones  cease  to  rise 
or  creep  in  front  of  the  roller.  The  second  course  then  is  spread  to 
a  depth  of  about  4  inches,  rolled,  sprinkled,  and  again  rolled  in  the 
same  manner  and  to  the  same  extent  as  the  first  course.  After  this, 
a  thin  coat  of  the  fine  screenings  is  spread  over  the  surface  and  the 
traffic  is  admitted. 

If,  during  the  rolling,  the  first  course  appears  to  be  deficient  in 
binding  material,  more  may  be  added  by  spreading  a  thin  layer  of 
the  fine  material  over  the  surface  of  the  course,  sprinkling  and 
rolling,  as  above  described. 

If,  during  the  rolling  of  the  top  course,  any  stones  larger  than 
1|  inches  appear,  they  must  be  removed. 

Gravel  shrinks  in  rolling  about  20  per  cent  of  its  loose  depth; 
therefore,  to  obtain  a  thickness  of  8  inches  when  compacted,  the 


HIGHWAY  CONSTRUCTION  85 

loose  material  should  have  a  depth  of  about  10  inches.  The  thick- 
ness of  the  gravel  coating  varies  according  to  the  nature  of  the 
roadbed,  a  thicker  layer  being  necessary  on  impermeable  soil  than 
on  a  well-drained  soil. 

The  pebbles  in  a  gravel  road  are  imbedded  in  a  paste  and  can  be 
displaced  easily.  It  is  for  this  reason,  among  others,  that  such 
roads  are  subject  to  internal  destruction. 

The  binding  power  of  clay  depends  in  a  large  measure  upon 
the  state  of  the  weather.  During  rainy  periods  a  gravel  road 
becomes  soft  and  muddy,  while  in  very  dry  weather  the  clay  will 
contract  and  crack,  thus  releasing  the  pebbles,  and  causing  a  loose 
surface.  The  most  favorable  conditions  are  obtained  in  moderately 
damp  or  dry  weather,  during  which  a  gravel  road  offers  several 
advantages  for  light  traffic,  the  character  of  the  drainage,  etc., 
largely  determining  durability,  cost,  maintenance,  etc. 

Repair.  Gravel  roads  constructed  as  above  described  will  need 
only  small  repairs  for  some  years,  but  daily  attention  is  required  in 
making  these.  A  garden  rake  should  be  kept  at  hand  to  draw  any 
loose  gravel  into  the  wheel  tracks,  and  for  filling  any  depressions 
that  may  occur. 

In  making  repairs,  it  is  best  to  apply  a  small  quantity  of  gravel 
at  a  time,  unless  it  is  a  spot  which  actually  has  cut  through.  Two 
inches  of  gravel  at  once  is  more  profitable  than  a  larger  amount. 
Where  a  thick  coating  is  applied  at  once  it  does  not  all  pack,  and  if, 
after  the  surface  is  solid,  a  cut  be  made,  loose  gravel  will  be  found; 
this  holds  water  and  makes  the  road  heave  and  become  spouty 
under  the  action  of  frost.  It  will  cost  no  more  to  apply  6  inches  of 
gravel  at  three  different  times  than  to  do  it  at  once. 

At  every  J  mile  a  few  cubic  yards  of  gravel  should  be  stored  to 
be  used  in  filling  depressions  and  ruts  as  fast  as  they  appear,  and 
there  should  be  at  least  one  laborer  to  every  5  miles  of  road. 

Broken=Stone  Roads 

Methods  of  Construction.  Broken-stone  roads  are  formed  in 
several  different  ways.  For  example,  the  road  may  be  formed  by 
placing  one  or  two  layers  of  stone  broken  into  small  fragments 
upon:  (1)  the  natural  soil;  (2)  a  foundation  composed  of  large  stone 
set  by  hand  upon  the  natural  soil ;  or  (3)  a  foundation  layer  of  cement 


86 


HIGHWAY  CONSTRUCTION 


HIGHWAY  CONSTRUCTION  87 

concrete.  The  layers  of  broken  stone  are  compacted  by  rolling 
with  a  heavy  roller  and  the  interstices,  or  spaces  between  the  stones 
are  filled  with  a  binder  composed  either  of  stone  dust;  stone 
dust  and  clay;  a  grout  of  hydraulic  or  Portland  cement;  or  a  bitu- 
minous cement  derived  from  either  coal  tar  or  asphalt,  and  used 
alone  or  mixed  with  sand.  The  broken  stone  forming  the  lower 
surface  layer  often  is  coated  with  a  bituminous  cement  before  placing 
it  upon  the  foundation.  This  applies  particularly  when  the  upper 
layer  is  of  bituminous  cement.  The  broken  stone  also  may  be 
mixed  with  Portland  cement  and  sand,  forming  a  concrete,  which 
is  placed  either  upon  the  prepared  natural  soil  or  upon  a  concrete 
or  broken-stone  foundation. 

The  several  methods  for  constructing  broken-stone  roads  are 
distinguished  by  either  a  specific  name  or  the  name  of  the  introducer. 
Thus,  the  types  known  as  Telford  and  Macadam  are  named 
from  Thomas  Telford  and  John  L.  McAdam,  Scottish  engineers, 
who  introduced  them  in  England  during  the  early  part  of  the  19th 
Century,  as  an  improvement  of  the  method  employed  in  the  18th 
Century  by  M.  Tresaguet  on  the  roads  of  France.  Telford  used  a 
base  of  large  stones,  Fig.  61,  upon  which  the  small  stone  was  placed. 
McAdam  omitted  the  base  contending  that  it  was  useless  and 
injurious.  Both  constructors  insisted  on  thorough  drainage  of  the 
subsoil,  but  neither  used  a  binder  and  rolling  was  unknown.  The 
stones  were  left  to  be  compacted  by  the  traffic.  The  introduction  of 
stone-crushing  machinery  and  rollers  as  well  as  the  practice  (con- 
demned by  McAdam,  but  advocated  by  Mr.  Edgeworth,  an  Irish 
landowner  in  his  treatise  on  Road  Building  published  in  1817)  of  filling 
the  voids  with  a  binder  has  caused  material  departures  from  the 
methods  of  the  pioneers  whose  names  are  still  but  improperly  applied. 

The  cement  grouting  was  introduced  in  England  by  Sir  John 
Macneil.  The  coating  of  the  stone  with  coal  tar  was  first  prac- 
ticed in  England  about  1840,  and  was  called  "tar-macadam".  In 
recent  times,  to  distinguish  the  several  varieties  of  bituminous  con- 
struction, several  specific  terms  have  been  coined,  as  "bitulithic", 
"tarmac",  "warrenite",  "bituminous  macadam",  "asphalt  macadam", 
etc.  Since  the  use  of  bituminous  binding  has  become  extensive,  the 
term  "water-bound  macadam"  has  come  into  use,  to  distinguish  the 
earlier  macadam  type  from  the  types  recently  introduced. 


88  HIGHWAY  CONSTRUCTION 

Quality  of  Stones.  The  materials  used  for  broken-stone  pave- 
ments of  necessity  must  vary  very  much  according  to  the  locality. 
Owing  to  the  cost  of  haulage,  local  stone  generally  must  be  used, 
especially  if  the  traffic  be  only  moderate.  If,  however,  the  traffic  is 
heavy,  it  sometimes  will  be  found  better  and  more  economical  to 
obtain  a  superior  material,  even  at  a  higher  cost,  than  the  local 
stone;  and  in  cases  where  the  traffic  is  very  great,  the  best  material 
that  can  be  obtained  is  the  most  economical. 

There  are  a  number  of  qualities  required  in  a  stone  to  render 
efficient  service.  Hardness  and  toughness,  to  resist  the  effects  of 
abrasion  and  impact.  These  two  properties,  while  closely  related, 
are  not  always  coincident;  some  rocks,  although  extremely  hard,  yet 
are  so  brittle  that  they  crush  easily  under  impact.  In  others  the 
cohesion  between  the  component  particles  is  so  weak  that  they  are 
worn  quickly  by  abrasion.  Durability,  or  power  to  resist  the  disin- 
tegrating influences  of  the  weather  and  humus  acids.  The  quality 
of  durability  depends  chiefly  upon  the  chemical  stability  of  the 
minerals  present.  Physical  defects  and  abrasion  generally  cause 
the  destruction  of  the  stone  long  before  it  is  injured  by  chemical 
changes.  Capability  of  binding  into  a  compact  mass.  This  quality 
is  essential  to  stone  used  for  water-bound  macadam.  The  binding 
or  cementing  property  is  possessed  to  a  greater  or  less  extent  by  all 
rocks  when  in  a  state  of  disintegration.  It  is  caused  by  the  action 
of  water  upon  the  chemical  constituents  of  the  stone  contained  in 
the  detritus — material  worn  off — produced  by  crushing  the  stone, 
and  by  the  friction  of  the  fragments  on  each  other  while  being  com- 
pacted; its  strength  varies  with  the  different  species  of  rock,  but  it 
exists  in  some  measure  with  them  all,  being  greatest  with  limestone 
and  least  with  gneiss. 

The  essential  condition  of  the  stone  to  produce  this  binding 
effect  is  that  it  be  sound.  No  decayed  stone  retains  the  property  of 
binding,  though  in  some  few  cases,  where  the  material  contains  iron 
oxides,  it  may,  by  the  cementing  property  of  the  oxide,  undergo  a 
certain  amount  of  binding. 

A  stone  of  good  binding  nature  frequently  will  wear  much 
better  than  one  without,  although  it  is  not  so  hard.  A  limestone 
road  well  made  and  of  good  cross  section  will  be  more  impervious 
than  any  other,  owing  to  this  cause,  and  will  not  disintegrate  so 


HIGHWAY  CONSTRUCTION  89 

soon  in  dry  weather,  owing  partly  to  this  and  partly  to  the  well- 
known  quality  which  all  limestone  has  of  absorbing  moisture  from 
the  atmosphere.  Mere  hardness  without  toughness  is  not  of  much 
use,  as  a  stone  may  be  very  hard  but  so  brittle  as  to  be  crushed  to 
powder  under  a  heavy  load,  while  a  stone  not  so  hard  but  having  a 
greater  degree  of  toughness  will  be  uninjured. 

A  stone  for  a  road  surface  should  be  as  little  absorptive  of 
moisture  as  possible  in  order  that  it  may  not  suffer  injury  from  the 
action  of  frost.  Many  limestones  are  objectionable  on  this  account. 

The  stone  used  should  be  uniform  in  quality,  otherwise  it  will 
wear  unevenly,  and  depressions  will  appear  where  the  softer  material 
has  been  used.  As  the  under  parts  of  the  road  covering  are  not 
subject  to  the  wear  of  traffic,  and  have  only  the  weight  of  loads  to 
sustain,  it  is  not  necessary  that  the  stone  of  the  lower  layer  be  so 
hard  or  so  tough  as  the  stone  for  the  surface,  hence  it  is  frequently 
possible  by  using  an  inferior  stone  for  that  portion  of  the  work,  to 
reduce  greatly  the  cost  of  construction. 

Testing  the  Rock.  In  order  to  ascertain  the  probable  resist- 
ance of  the  different  rocks  to  the  destructive  action  of  the  traffic  and 
weather,  tests  are  made  in  the  laboratory  to  determine  the  resistance 
to  impact  and  abrasion,  absorptive  capacity,  hardness,  toughness, 
and  specific  gravity. 

Abrasion.  The  test  for  abrasion  is  conducted  in  the  Deval  type 
of  machine.  It  consists  of  two  or  more  cast-iron  cylinders  mounted 
on  a  shaft  so  that  the  axis  of  each  cylinder  is  inclined  an  angle  of  30 
degrees  from  the  axis  of  rotation.  The  cylinders'  are  charged  with 
11  pounds  of  the  rock  broken  into  fragments,  ranging  from  1J  to  2| 
inches.  The  cylinders  are  then  rotated  at  a  uniform  speed  of  2,000 
revolutions  per  hour  for  five  hours,  or  until  the  automatic  recorder 
shows  10,000  revolutions;  the  charge  then  is  removed  and  placed 
on  a  sieve  having  meshes  of  •£$  inch.  The  material  retained  on  the 
sieve  is  washed,  dried,  and  weighed.  The  difference  in  weight 
between  the  weight  of  the  charge  and  the  residue  larger  than  &  inch 
shows  the  loss  by  abrasion. 

Impact  and  Toughness.  The  test  for  impact  and  toughness  is 
made  in  a  machine,  consisting  of  an  anvil,  plunger,  and  hammer, 
mounted  in  vertical  guides.  The  test  piece  is  placed  on  the  anvil; 
the  hammer  weighing  4.40  pounds  is  raised  and  allowed  to  fall  a 


90  HIGHWAY  CONSTRUCTION 

distance  of  one  centimeter  for  the  first  blow  and  an  increased  fall  of 
one  centimeter  for  each  succeeding  blow,  until  the  test  piece  fails. 
The  number  of  blows  required  to  destroy  it  is  used  to  represent  the 
toughness;  13  blows  is  considered  to  indicate  low  resistance,  13  to 
19  medium,  and  above  19  high. 

Hardness.  The  test  for  hardness  is  made  on  a  Dory  machine, 
which  consists  of  a  steel  disk  mounted  so  as  to  be  revolved.  The 
test  pieces  are  cylinders  cut  from  the  rock  by  a  core  drill,  and  the 
ends  ground  level.  Two  pieces  are  used  for  a  test;  each  is  weighed, 
then  placed  in  the  guides  of  the  machine  with  its  face  resting  upon 
the  grinding  disk.  The  machine  is  revolved  until  1000  revolutions 
have  been  made,  and  during  the  operation,  quartz  sand  is  fed  onto 
the  disk.  The  test  piece  is  removed  and  weighed,  and  the  hardness 
is  determined  from  the  formula 

W 

Hardness  =  20  —  — 
o 

in  which  W  is  loss  in  grams  per  1000  revolutions.  Rocks  having  a 
hardness  less  than  14  are  considered  soft;  from  14  to  17  medium; 
and  over  17  hard. 

Water  Absorption.  The  capacity  of  the  stone  to  absorb  water 
is  determined  by  using  a  thoroughly  dry  sample  of  stone  weighing 
about  12  grams.  The  sample  is  weighed  in  air,  then  immersed  in 
water  where  it  is  weighed  immediately;  after  96  hours'  immersion  it 
is  weighed  again  in  the  water.  The  absorptive  capacity  then  is 
calculated  by  the  formula 

/>  _    T> 

Lb.  water  absorbed  =  —  —  -  X62.37  per  cu.  ft.  of  rock 


in  which  A  is  the  weight  in  air;  B  is  the  weight  in  water  immediately 
after  immersion;  C  is  the  weight  in  water  after  immersion  for  96 
hours;  and  62.37  is  the  normal  weight  in  pounds  of  a  cubic  foot  of 
water. 

The  durability  of  a  stone  used  for  roads  is  affected  to  a  certain 
extent  by  its  capability  of  absorbing  water.  In  cold  climates  a  low 
absorptive  capacity  is  essential  to  resist  the  disintegrating  effects 
of  alternate  freezing  and  thawing. 

Specific  Gravity.    The  specific  gravity  is  determined  either  by 


HIGHWAY  CONSTRUCTION  91 

weighing  in  a  specific  gravity  balance   or  by  weighing  in  air  and 
water,  and  applying  the  formula 

Specific  gravity  =  j^rr^r 

in  which  W  is  the  weight  in  air,  and  W\  is  the  weight  in  water. 

Specific  gravity  and  porosity  are  closely  related.  The  specific 
gravity  varies  with  the  density  or  compactness  of  the  aggregation  of 
the  mineral  grains  forming  the  stone.  The  closer  the  grains  the  more 
compact  the  stone,  and  the  less  will  be  the  amount  of  interstitial 
space  and  hence  the  less  the  porosity. 

From  the  specific  gravity  the  weight  per  ton  or  per  cubic  yard 
may  be  determined.  A  knowledge  of  the  weight  is  useful  in  decid- 
ing between  two  otherwise  good  stones;  the  heavier  will  be  the  more 
expensive,  due  to  increased  cost  of  transportation.  On  a  water- 
bound  macadam  road  it  is  an  advantage  to  have  a  detritus  with  a 
high  specific  gravity,  as  it  will  not  be  moved  so  easily  by  rain  and 
wind  as  one  of  low  specific  gravity. 

Cementing  Quality.  The  cementing  quality  of  the  stone  dust  is 
determined  by  placing  500  grams  of  the  rock,  broken  to  pass  a  J-inch 
mesh  screen,  in  a  ball  mill,  together  with  90  cubic  centimeters  of 
water  and  2  steel  balls  weighing  20  pounds.  The  mill  and  its  charge 
are  revolved  for  2J  hours  at  a  rate  of  2000  revolutions  per  hour. 
The  operation  produces  a  stiff  dough,  of  which  25  grams  are  placed 
in  a  metal  die  25  millimeters  in  diameter,  and  subjected  to  a  pressure 
of  132  kilograms  per  square  centimeter,  producing  a  cylindrical  test 
piece.  The  test  piece  is  dried  in  the  air  for  20  hours,  after  which  it 
is  heated  in  a  hot-air  oven  for  4  hours  at  a  temperature  of  200° 
Fahrenheit  and  then  cooled  in  a  desiccator  for  20  minutes.  When 
cool  it  is  tested  in  the  impact  testing  machine  in  the  same  manner 
as  the  test  for  toughness,  using  a  hammer  weighing  1  kilogram  and 
a  fixed  height  of  fall  of  1  centimeter.  Blows  are  struck  until  the 
test  piece  fails.  The  average  of  the  number  of  blows  on  5  test 
pieces  is  taken  as  the  result  of  the  test.  A  result  of  10  is  considered 
to  indicate  a  low  cementing  quality;  10  to  25  is  considered  fair;  26 
to  75  good;  76  to  100  very  good;  over  100  excellent. 

Species  of  Stone.  The  rocks  most  extensively  used  for  broken- 
stone  roads  are  trap,  granite,  limestone,  sandstone,  boulders,  or  field- 
stone. 


92  HIGHWAY  CONSTRUCTION 

Trap  rock  is  hard  and  tough  and  has  good  wearing  and  binding 
qualities.  Granite  is  brittle  and  its  cementing  value  is  low.  Lime- 
stone is  deficient  in  hardness  and  toughness  but  possesses  good 
binding  qualities  and  for  light  traffic  roads  is  eminently  suitable. 
Sandstones  are  rocks  made  up  of  grains  of  sand  cemented  together 
by  siliceous,  ferruginous,  calcareous,  or  argillaceous  material;  they 
are  usually  deficient  in  binding  quality  and  resistance  to  abrasion. 
With  a  bituminous  binder  good  results  are  obtained.  Fieldstones 
are  a  mixture  of  the  hardest  parts  of  the  granites,  sandstones,  lime- 
stones, etc.,  distributed  by  glacial  action  and  which  have  resisted 
the  disintegrating  effects  of  the  weather.  Owing  to  their  variable 
character  and  unequal  hardness  they  wear  irregularly  and  make  a 
very  rough  road. 

Shape  and  Size  of  Stone.  The  shape  and  size  of  the  fragments 
of  stone  affect  the  enduring  qualities  of  the  road.  The  nearer  the 
fragments  approach  the  cubical  form  with  irregular  jagged  sides, 
the  more  satisfactory  will  be  the  results. 

The  size  of  the  stone  must  be  such  as  will  not  fracture  or  crush 
under  the  action  of  the  roller  during  compaction  nor  become  loosened 
under  traffic.  For  the  harder  rocks  the  size  varies  from  1J  to  2| 
inches  and  for  the  softer  rocks  from  2J  to  4  inches.  The  sizes  do 
not  refer  to  the  actual  dimensions  of  the  stone,  but  to  the  size  of  the 
hole  in  the  screen;  thus  IJ-inch  stone  is  that  which  is  retained  by  a 
1^-inch  opening  and  passes  through  a  2-  or  2J-inch  opening. 

Thickness  of  the  Broken  Stone.  The  thickness  of  the  broken 
stone  is  determined  for  the  given  conditions  by  the  formula  pre- 
viously stated,  and  ranges  from  4  to  16  inches.  Where  the  thickness 
exceeds  6  inches,  the  excess  may  be  composed  of  sand,  gravel,  field- 
stone,  ledge  rock,  or  broken  stone,  as  previously  stated  in  the  dis- 
cussion on  foundations;  the  choice  depending  on  availability  and 
cost.  For  use  in  the  base  all  are  equally  effective. 

Spreading  the  Stone.  The  method  employed  for  laying  the 
covering  varies  with  the  thickness.  When  the  finished  thickness  is 
4  inches  all  the  stone  to  be  used  is  laid  in  one  course.  When  the 
finished  thickness  exceeds  4  inches  the  stone  is  laid  in  two  or  more 
courses;  the  top  or  wearing  course,  being  composed  of  the  best  and 
most  expensive  stone,  is  made  the  least  thickness  compatible  with 
good  construction  and  maintenance.  To  provide  for  the  shrinkage 


HIGHWAY  CONSTRUCTION 


93 


of  the  stone  under  the  roller,  the  depth  of  the  courses  of  loose  stone 
should  exceed  the  finished  depth  by  from  25  to  30  per  cent. 

The  stone  is  hauled  upon  the  roadbed  in  vehicles  of  various 
types  provided  with  broad-tired  wheels.  In  some  types  of  vehicle 
it  is  spread  in  layers  as  the  vehicle  is  drawn  along  the  roadbed ;  with 
others  it  is  dumped  in  heaps  and  spread  by  hand  with  forks  and 
brought  to  an  even  surface  by  raking,  Fig.  62. 

Compacting  the  Broken  Stone.  The  stone  is  compacted  by 
rolling  with  heavy  rollers  drawn  by  horses  or  propelled  by  steam  or 
other  power,  Fig.  63.  The  steam  roller  is  more  effective  than  horse- 


Fig.  62.     View  Showing  Spreading  of  Lower  Course  of  Macadam  Road 
Courtesy  of  United  States  Department  of  Agriculture 

drawn  rollers.  The  usual  weights  of  steam  rollers  are  5,  10,  and  15 
tons;  the  10-ton  being  the  one  generally  used,  although  the  weight 
of  the  roller  should  be  selected  in  accordance  with  the  bearing  power 
of  the  natural  soil.  A  roller  having  excessive  weight  may  cause 
injury  to  the  roadbed,  by  rolling  it  into  undulations  that  will  permit 
water  to  collect  and  consequently  cause  damage.  A  roadbed  which 
will  stand  a  heavy  roller  in  dry  weather  may  be  injured  by  it  during 
wet  weather.  For  a  weak  roadbed  it  is  well  to  use  two  rollers,  one  of 
light  weight  to  form  a  crust  and  a  narrow,  heavy  roller  to  compact  it. 


94 


HIGHWAY  CONSTRUCTION 


The  roller  should  commence  at  one  edge  or  border  of  the  road- 
way, and  move  along  that  edge  until  within  about  25  feet  of  one 
end  of  the  spread  stone;  it  then  should  cross  over  to  the  other  edge 
and  proceed  along  this  edge  to  the  beginning,  crossing  over  and 
overlapping  the  strips  previously  rolled  until  the  center  of  the  road 
is  reached.  The  rolling  is  continued  in  this  manner  until  the  stones 
cease  to  creep  in  front  or  sink  under  the  roller.  If,  during  the  first 
passages  of  the  roller,  low  spots  appear,  they  should  be  filled  to 
grade  with  stone  of  the  same  size  as  is  in  the  course  being  rolled. 


Fig.  63.     Compacting  Broken  Stone  by  Steam  Roller 
Courtesy  of  United  States  Department  of  Agriculture 

After  about  two  passages  of  the  roller,  the  binder,  consisting  of 
the  screenings  from  the  stone  being  used  for  the  course,  is  spread  in 
a  thin  layer  over  the  surface  of  the  partly  compacted  stone  and 
sprinkled  with  water,  which  washes  it  into  the  voids  in  the  stone; 
the  rolling  then  is  continued,  Fig.  64.  The  operation  of  applying 
the  binder,  sprinkling,  and  rolling  is  repeated  until  a  wave  of  water 
and  screenings  rises  in  front  of  the  roller.  Each  course  is  treated 
and  rolled  in  the  same  manner.  If  the  screenings  from  the  rock 
that  is  being  used  are  not  suitable  for  binding,  screenings  from  other 
rock,  clay,  sand,  or  loam  are  substituted. 


HIGHWAY  CONSTRUCTION 


95 


An  excess  of  binder  and  water  will  shorten  the  time  required  to 
consolidate  the  stone  and  produce  the  appearance  of  a  good  piece  of 
work,  but  under  traffic  it  will  wear  unevenly  and  go  to  pieces  quickly. 

Suppression  of  Dust  on  Macadam  and  Telford  Roads.  Since 
the  introduction  of  mechanically ,  propelled  vehicles,  broken-stone 
roads  constructed  according  to  the  principles  of  Telford  and  McAdam, 
have  proven  inadequate  to  the  demands  of  the  changed  traffic. 

The  adhesion  between  the  particles  of  stone  is  insufficient  to 
react  against  the  propulsive  force  exerted  by  the  driving  wheels, 


Fig.  64.     Rolling  and  Sprinkling  Second  Course  of  Macadam  Road  to  Complete 

Binding  Process 
Courtesy  of  United  States  Department  of  Agriculture 

hence  the  stones  are  loosened,  and  although  the  rubber  tires  with 
which  the  motor  vehicles  are  equipped  produce  little  dust  by  attri- 
tion or  wearing  away,  the  vehicle  moving  at  high  speed  creates  a 
partial  vacuum.  The  current  of  air  which  then  rushes  in  to  re-estab- 
lish the  equilibrium  picks  up  the  small  particles  of  stone  displaced 
and  loosened  by  the  thrust  of  the  driving  wheels  and  distributes 
them  in  the  form  of  dust,  which  is  very  disagreeable  to  other  users 
of  the  road  and  residents  along  it.  The  large  stones  that  are  loosened 


96  HIGHWAY  CONSTRUCTION 

are  thrown  about  and  ground  upon  one  another  and  thus  increase 
the  amount  of  fine  material  ready  to  be  scattered  as  dust. 

The  frequent  repetition  of  these  actions  causes  the  pavement 
to  pit  and.  disintegrate.  The  destructive  effect  is  intensified,  the 
greater  the  speed,  and  where  the  irregularities  of  the  surfaces  are 
such  as  to  cause  the  wheels  to  leave  it,  there  is  produced  a  bounding 
motion  that  is  continued  for  some  distance  and  is  particularly  dis- 
astrous. Shearing  of  the  road  fabric  is  very  severe  on  steep  grades 
and  curves  due  to  the  slipping  of  the  driving  wheels  when  the  pro- 
pulsive force  is  greater  than  the  adhesion  between  the  tire  and  the 
road  surface.  The  damage  arising  from  this  is  more  extensive 
during  wet  weather  and  is  intensified  when  the  wheels  are  equipped 
with  bars,  chains,  studs,  and  other  anti-skidding  devices. 

The  formation  bf  dust  and  mud  cannot  be  prevented  absolutely, 
because  all  materials,  by  attrition  and  the  disintegrating  action  of 
the  elements,  yield  dust  when  dry  and  mud  when  wet.  If  the  sur- 
face of  a  water-bound  macadam  road  could  be  maintained  in  a 
moist  condition,  there  would  be  no  dust,  but  moistening  with  water 
even  in  cities,  towns,  and  villages  is  expensive,  and  in  rural  districts 
the  cost  is  prohibitive  and  the  practice  would  be  impossible,  owing 
to  the  absence  of  water  available  for  the  purpose.  Hence  in  dealing 
with  existing  road  surfaces  a  remedy  has  been  sought  in  more  fre- 
quent cleansing  and  in  the  use  of  some  substitute  for  water  which 
would  be  cheap,  effective,  lasting,  and  easily  applied.  To  meet  this 
demand  several  "dust-laying"  compositions  have  been  placed  on 
the  market,  and  experiments  have  been  made  with  some  of  these, 
but  it  has  been  demonstrated  clearly  that,  with  but  few  exceptions, 
they  have  a  very  temporary  effect,  and  their  application  must  be 
frequent  and  thorough. 

Under  the  head  of  exceptions,  that  is,  of  the  more  or  less 
permanent  methods,  are  included  the  following:  (1)  the  cementing 
of  the  surface  stone  by  a  bituminous  cement  or  binder.  When 
the  binder  is  applied  by  the  penetration  method,  the  surface  is 
described  by  the  general  term  " bituminous-macadam";  and  when  it  is 
desired  to  indicate  the  kind  of  binder,  the  descriptive  names,  "asphalt- 
macadam",  "tar-macadam",  etc.,  are  used.  When  the  binder  is 
applied  by  the  mixing  method,  the  construction  is  called  "bitumi- 
nous-concrete", or  specifically  designated  by  the  trade  or  patented 


HIGHWAY  CONSTRUCTION  97 

name  as,  "bitulithic",  "warrenite",  "amiesite",  "filbertine",  "rock- 
asphalt",  etc.;  (2)  binding  the  stone  with  hydraulic  cement,  the 
surface  so  formed  being  called  "concrete-macadam",  or  "concrete 
pavement".  These  will  be  discussed  later  under  their  respective 
headings. 

Turning  to  the  details  of  the  various  temporary  methods,  we 
find  the  following : 

(1)  Fresh  Water.    This  is  the  simplest  remedy,  but  not  always 
the  most  practicable  nor  the  cheapest. 

(2)  Sea  Water.    This  is  a  simple  remedy  but  available  only  on 
the  seacoast.    The  salts  contained  in  sea  water  are  highly  anti- 
septic and  deliquescent;  a  light  sprinkling  will  suppress  the  dust  for 
several  hours.    Its  use,  however,  is  objected  to  for  the  reason  that 
it  injures  the  varnish  and  running  gear  of  vehicles,  corrodes  cast- 
iron  street  fittings,  and  when  the  road  surface  on  which  it  has  been 
used  has  dried  the  dust  then  produced,  containing  salt,  injures  food 
and  other  goods  exposed  to  it.    Moreover,  after  a  few  weeks'  use 
the  dust  is  converted  into  a  pasty  mud  that  adheres  to  the  wheels 
and  causes  the  surface  of  the  road  to  be  "picked  up". 

(3)  Deliquescent  Salts.    The  chief  advantage  of  these  salts  is 
that  their  effect  is  more  lasting  than  that  of  water.     The  salt  used 
most  extensively  is  calcium  chloride  obtained  as  a  by-product  in  the 
manufacture  of  soda  by  the  ammonia  process.    The  salt  may  be 
applied  either  in  solution  or  in  the  dry  form.     It  takes  up  water 
rapidly  and  proves  very  efficient  where  the  atmospheric  moisture  is 
sufficient  to  feed  the  salt.     Glutrin,  the  commercial  name  for  the 
waste  sulphite  liquor  obtained  in  the  manufacture  of  paper  from 
wood  pulp  by  the  sulphite  process,  reduces  the  formation  of  dust, 
but  the  treatment  must  be  repeated  frequently.     Waste  molasses 
or  "black  strap"  from  sugar  refineries  mixed  with  milk  of  lime 
possesses  good  dust-suppressing  qualities. 

(4)  Coal- Tar  Coating.    Refined  coal  tar  applied  either  hot  or 
cold  in  the  form  of  a  spray  minimizes  the  production  of  dust,  renders 
the  surface  waterproof,  and  reduces  wear.     The  success  attending 
its  use  depends  upon  the  quality  of  the  tar,  the  state  of  the  weather, 
which  must  be  clear  and  dry,  the  condition  of  the  road  surface, 
which  must  be  dry  and  free  from  dust  and  dirt,  and,  in  the  case  of 
hot  application,  that  the  tar  is  not  overheated. 


98  HIGHWAY  CONSTRUCTION 

(5)  Solutions  of  Coal  Tar  and  Petroleum.     Several  patented 
preparations  of  coal  tar  are  on  the  market.    The  principle  of  all  is 
practically  the  same,  namely,  the  solution  of  the  tar  or  oil  in  water 
by  a  volatile  agent,  which  on  evaporation  leaves  a  more  or  less 
insoluble  coating  on  the  road  surface.     The  more  favorably  known 
of  these  preparations  are  "tarvia"  and  "westrumite". 

(6)  Crude   Petroleum  and   Residuum  Oil.    Crude  petroleum 
containing  a  large  percentage  of  asphalt  gives  the  best  results. 
Petroleum  having  paraffin  and  naphtha  as  a  base  refuses  to  bind, 
and  produces  a  greasy  slime.    The  residuum  oils  obtained  in  the 
distillation  of  petroleum  having  asphaltum  for  a  base  have  yielded 
good  results  in  many  cases. 

Two  methods  are  followed  in  applying  the  oil:  (a)  The  sur- 
face of  the  road  to  be  oiled  is  prepared  by  removing  the  dust  with 
hand  or  power  brooms.  The  oil,  in  the  cold  method,  is  applied  by 
specially  designed  sprinkling  wagons,  at  the  rate  of  from  one-third 
to  one-half  gallon  per  square  yard.  After  being  applied  the  oil  is 
covered  with  sand  or  stone  screenings  and  may  or  may  not  be 
rolled.  The  oil  is  applied  once  or  twice  a  year  according  to  whether 
the  traffic  is  light  or  heavy.  The  surface  of  the  road  must  be  dry 
when  the  oil  is  applied. 

(b)  The  oil  is  sprinkled  over  the  surface  and  mixed  with  the 
dust.  If  the  oil  is  merely  sprinkled,  the  mixture  of  dust  and  oil 
made  by  the  action  of  the  traffic  will  become  very  sticky  and  will  be 
removed  in  spots  by  adhering  to  the  wheels.  For  the  purpose  of 
facilitating  the  handling  and  of  securing  a  deeper  penetration 
than  is  possible  with  cold  oil,  the  oil  is  heated  to  a  temperature  of 
about  140°  Fahrenheit  and  applied  in  the  same  manner  as  the 
cold  oil. 

(7)  Oil  Tar  and  Creosote.    Oil  tar  is  the  residual  liquid  from 
the  manufacture  of  carbureted  water  gas  and  oil  gas.     The  tar  used 
for  road  purposes  is  obtained  by  distilling  the  original  tarry  liquid  to 
remove  the  light  oil,  naphthalene,  and  creosote.     Various  grades  of 
tar  are  produced  according  to  the  temperature  at  which  the  dis- 
tillation is  stopped.    The  higher  the  temperature  of  distillation,  the 
harder  and  more  brittle  the  tar. 

The  oil  tar  either  alone  or  mixed  with  creosote  is  applied  in  the 
same  manner  as  coal  tar. 


HIGHWAY  CONSTRUCTION  99 

Bituminous=Macadam 

Features  of  Bituminous=Macadam.  A  bituminous-macadam 
wearing  surface  differs  from  the  previously  described  water-bound 
broken-stone  surface  only  in  the  kind  of  binder  and  the  quality 
of  the  stone.  The  bituminous  binder  is  prepared  from  asphalt, 
asphaltic  oils,  refined  water-gas  tars,  refined  coal  tars,  and  com- 
binations of  refined  tars  and  asphalts. 

The  bituminous  binders  adhere  to  comparatively  porous  and 
relatively  soft  stone,  such  as  limestone,  better  than  to  the  hard 
stones,  such  as  trap  and  granite.  Consequently,  the  stone  used 
with  the  bituminous  binders  may  be  inferior  in  hardness  and  binding 
quality  to  that  required  for  water-bound  macadam. 

Methods  of  Construction.  The  essentials  necessary  to  the 
successful  construction  of  a  bituminous  covering  are:  (1)  the  exclu- 
sion of  both  subsoil  and  surface  water  from  the  foundation;  (2)  a 
solid  unyielding  foundation;  (3)  a  stone  of  suitable  quality  and 
size;  (4)  that  the  stone  shall  be  entirely  free  from  dust,  otherwise 
the  dust  will  interpose  a  thin  film  between  the  stone  and  the  bitu- 
minous binder  and  prevent  the  latter  from  adhering  to  the  stone; 
(5)  if  the  stone  is  to  be  used  hot,  that  it  shall  not  be  overheated; 
and  if  is  to  be  used  cold,  that  it  shall  be  dry,  for  if  wet  or  damp, 
the  bituminous  material  will  not  adhere  to  it;  (6)  that  the  bituminous 
cement  shall  be  of  suitable  quality;  free  from  water,  for  which  the 
stone  has  a  greater  affinity  than  for  bitumen,  and  would  thus  pre- 
vent adhesion;  free  from  ammoniacal  liquor,  which  is  apt  to  saponify 
some  of  the  oily  constitutents  and  thus  render  them  capable  of 
combining  with  water  and  therefore  apt  to  be  washed  out;  free 
from  an  excess  of  light  oils  and  naphtha,  which  act  as  diluents 
and  volatilize  on  the  surface  of  the  road,  forming  a  skin  that  is 
not  durable;  free  from  an  excess  of  free  carbon,  because  it  has  no 
binding  value  and  is  liable  to  be  converted  into  dust  and  mud. 

Two  general  methods  with  various  modifications  in  the  minor 
details  are  employed  for  applying  the  bituminous  binder  to  form 
the  wearing  surface,  viz,  the  penetration  method,  and  the  mixing 
method. 

Penetration  Method.  In  this  method,  the  stone  is  spread  and 
packed  slightly  by  rolling.  The  bituminous  binder  is  then  applied 
by  one  of  the  following  ways:  by  hand  from  pouring  pots;  by  a 


100 


HIGHWAY  CONSTRUCTION 


nozzle  leading  from  a  tank  cart;  or  by  a  mechanical  distributor  using 
air  pressure  to  discharge  the  material  through  nozzles  that  spread 
it  in  a  finely  divided  stream  or  spray,  Fig.  65.  The  binder  is  heated, 
usually  by  steam  from  the  roller,  but  when  hand  pots  are  used,  it 
is  heated  in  kettles  over  fires.  The  quantity  applied  is  about  If 
gallons  per  square  yard.  After  the  binder  is  distributed,  it  is  covered 

with  a  light  coating  of  stone 
dust,  sand,  or  gravel,  and  the 
rolling  is  continued.  In  some 
cases,  after  the  rolling  is  com- 
pleted, another  application  of 
the  binder  is  made  at  the  rate 
of  about  one-half  gallon  per 
square  yard;  this  is  called  a 
"paint  coat"  and  is  covered 
with  a  light  sprinkling  of  stone 
screenings. 

Mixing  Method.  In  this 
method  the  stone  to  be  used 
for  the  wearing  surface,  vary- 
ing in  size  from  J  to  1J 
inches,  is  cleaned  and  dried, 
then  mixed  with  a  sufficient 
quantity  of  the  binder  to  coat 
all  the  stones  thoroughly. 
The  mixing  is  performed  by 
manual  labor  on  a  mixing 
board,  Fig.  66,  or  by  raking 
the  stones  through  a  bath  of 
liquid  binder,  or  by  passing 
through  a  mechanical  mixing 
machine,  Fig.  67.  The  coated  stones  are  spread  upon  the  foundation 
in  a  layer  having  a  thickness  of  about  3  inches  and  are  covered 
with  a  light  coating  of  stone  screenings  free  from  dust;  then  are 
compacted  by  rolling,  Fig.  68.  Wherever  the  binder  flushes  to 
the  surface  it  is  covered  with  screenings  and  rolled.  When  the 
rolling  is  completed,  the  surplus  screenings  are  swept  from  the 
surface.  The  cleaned  surface  then  is  covered  with  a  coat  of  the 


Fig.  65.     Spreading  Bituminous  Binder  by  Pressure 

Nozzle,  Penetration  Method 

Courtesy  of  Barrett  Manufacturing  Company, 

New  York  City 


HIGHWAY  CONSTRUCTION  101 


Fig.  66.     Hand  Method  of  Mixing  Stone  and  Binder 
Courtesy  of  Barrett  Manufacturing  Company,  New  York  City 


Fig.  67.     Machine  Method  of  Mixing  Stone  and  Binder 
Courtesy  of  Barrett  Manufacturing  Company,  New  York  City 


Fig.  68.     Rolling  Bituminous  Macadam  Road  Surface 
Courtesy  of  Barrett  Manufacturing  Company,  New  'York  City 


102 


HIGHWAY  CONSTRUCTION 


binder  called  a  "seal  coat",  for  the  purpose  of  insuring  the  water- 
proofing and  complete  filling  of  the  voids,  Fig.  69.  For  this  coat, 
about  one-half  gallon  of  binder  is  used  per  square  yard  of  surface. 
Screenings  again  are  spread  and  may  or  may  not  be  rolled. 

Advantages  and  Disadvantages  of  the  Penetration  Method.  The 
advantage  of  the  penetration  method  is  the  ease  and  rapidity  with 
which  it  can  be  carried  out,  and  the  low  cost  for  equipment  and  labor. 

The  disadvantages  of  the  penetration  method  are:  (1)  the 
difficulty  of  obtaining  an  absolutely  uniform  distribution  of  the  binder, 
thus  producing  "lean"  and  "fat"  spots  that  will  prove  defective  under 
traffic;  (2)  it  is  wasteful,  in  that  it  is  necessary  to  use  more  binder 
than  actually  is  required  to  coat  the  stones  and  bind  them  together; 


7T\ 


Fig.  69.     Spraying  Seal  Coat  by  Auto  Truck,  One-Half  Gallon  to  the  Yard 
Courtesy  of  Barrett  Manufacturing  Company,  New  York  City 

(3)  it  is  difficult  and  sometimes  impossible  to  use  a  binder  of  suffi- 
cient original  consistency  to  produce  a  satisfactory  bond,  owing  to 
the  bitumen  setting  too  rapidly  when  applied  to  cold  stone. 

Advantages  and  Disadvantages  of  the  Mixing  Method.  The 
advantages  of  the  mixing  method  are :  (1)  the  producing  of  a  uniform 
fabric  in  which  the  cement  is  distributed  uniformly  and  cements 
each  individual  stone;  (2)  that  construction  can  be  carried  on  in 
colder  weather  than  is  permissible  with  the  penetration  method. 
If  hot  stone  is  used,  a  bitumen  can  be  employed  of  such  original 
consistency  as  is  required  to  sustain  the  traffic  satisfactorily. 

The  disadvantage  of  the  mixing  method  is  the  greater  cost, 


HIGHWAY  CONSTRUCTION  103 

due  (1)  to  the  increased  labor,  and  (2)  to  the  more  elaborate  equip- 
ment and  apparatus  required. 

Bitulithic.  Bitulithic  is  composed  of  stone,  ranging  in  size  from 
2  inches  to  ^iU  of  an  inch,  and  dust,  which  are  dried,  heated, 
and  mixed  in  predetermined  proportions,  so  as  to  reduce  the  voids 
to  about  10  per  cent,  and  cemented  by  a  hot  bituminous  cement 
manufactured  from  either  coal  tar,  asphalt,  or  a  combination  of 
both.  The  cement  is  added  in  sufficient  quantity  not  only  to  coat 
every  particle  and  to  fill  all  of  the  remaining  voids  but  with  enough 
surplus  to  result  in  a  rubbery  and  slightly  flexible  condition  of  the 
mixture  after  compression. 

The  mixture  is  spread,  while  hot,  to  such  depth  as  will  give 
a  thickness  of  2  inches  after  compressing  with  a  10-ton  roller. 
After  rolling,  a  composition  coating  called  a  "flush  coat"  is  spread 
over  the  surface;  this  being  covered  while  sticky  with  hot  stone 
chips  which  are  rolled  until  cool.  The  purpose  of  the  stone  chips 
is  to  form  a  gritty  surface  to  prevent  slipping. 

Amiesite.  Amiesite  is  a  patented  preparation  of  crushed  stone 
or  gravel,  coated  writh  an  asphaltic  cement.  It  is  laid  in  two  courses 
and  a  surface  finish.  The  first  course, -composed  of  stone  ranging 
from  \  inch  to  \\  inches,  is  spread  to  a  depth  of  3  inches,  blocks 
or  strips  of  wood  being  used  to  insure  uniformity  of  depth,  then 
rolled  once.  The  second  course  is  composed  of  stone  \  inch  and 
less,  spread  1  inch  deep,  then  rolled.  The  surface  finish  consists 
of  screenings  or  sand,  used  in  sufficient  quantity  to  fill  the  voids. 

Rock  Asphalt.  The  rock  asphalt  most  used  in  the  United 
States  is  a  sandstone  containing  from  7  to  10  per  cent  of  asphalt. 
It  is  prepared  for  use  by  pulverizing  and  is  used  either  hot  or  cold. 
It  is  spread  upon  the  surface  of  the  stone  to  a  depth  of  about  \\ 
inches  and  rolled  with  a  steam  roller;  the  rolling  is  repeated  daily  for 
several  days,  or  until  the  asphalt  becomes  hard. 

Definitions  of  Bituminous  Materials.  The  most  recently 
adopted  definitions  of  the  bituminous  materials  employed  in  road 
construction  are: 

Native  Bitumen.  Native  bitumen  is  a  mixture  of  native  or 
pyrogenous  hydrocarbons  and  their  non-metallic  derivatives,  which 
may  be  gases,  liquids,  viscous  liquids,  or  solids  and  which  are  soluble 
in  carbon  disulphide. 


104  HIGHWAY  CONSTRUCTION 

Artificial  Bitumen.  Artificial  bitumen  is  produced  by  the 
destructive  distillation  of  pyrobitumens  and  other  substances  of 
an  organic  nature;  the  bitumens  so  produced  are  commonly  known 
as  tars,  the  word  tar  being  compounded  with  the  name  of  the  material 
which  has  been  subjected  to  the  process  of  destructive  distillation, 
thus  designating  its  origin,  as,  coal  tar,  oil  tar,  etc. 

Bituminous.  Bituminous  refers  to  that  which  contains  bitumen 
or  constitutes  a  source  of  bitumen. 

Emulsions.  Emulsions  are  oily  substances  made  mixable  with 
water  through  the  action  of  a  saponifying  agent  or  soap. 

Fixed  Carbon.  Fixed  carbon  is  the  organic  matter  of  the 
residual  coke  obtained  upon  burning  hydrocarbon  products  in  a 
covered  vessel  in  the  absence  of  free  oxygen. 

Fluxes.  Fluxes  are  fluid  oils  and  tars  which  are  incorporated 
with  asphalt  and  semi-solid  or  solid  oil  and  tar  residuums  for  the 
purpose  of  reducing  or  softening  their  consistency. 

Residuums,  Residual  Petroleum,  or  Residual  Oils.  These  are 
heavy  viscous  residues  produced  by  the  evaporation  or  distillation 
of  crude  petroleums  until  at  least  all  of  the  burning  oils  have  been 
removed. 

Bituminous  Cement.  The  bituminous  cements  or  binders 
are  prepared  from  (1)  coal-,  oil-,  and  water-gas  tars;  (2)  asphaltic 
petroleums;  (3)  asphalt;  and  (4)  combinations  of  asphalt  and  the 
residues  of  distillation  from  asphaltic  petroleums. 

Coal- Tar  Binder.  Coal-tar  binder  is  the  residue  obtained  by 
the  distillation  of  the  crude  tar  produced  in  the  manufacture  of 
illuminating  gas  and  the  manufacture  of  coke  for  metallurgical 
purposes;  the  required  consistency  is  obtained  by  removing  »part 
or  all  of  the  contained  oils.  Owing  to  the  difference  in  the  tem- 
peratures employed  in  the  two  producing  processes,  the  constituents 
of  the  tar,  while  identical  in  their  characteristics,  differ  in  their 
amount;  the  most  marked  difference  being  in  the  free  carbon  con- 
tent, of  which  coke-oven  tar  has  the  least. 

Oil-Gas  Tar.  Oil-gas  tar  is  produced  in  the  manufacture  of 
gas  from  oil.  The  tarry  residue  is  rather  varied  in  character  and 
is  prepared  for  use  by  distillation;  it  usually  contains  a  large  amount 
of  free  carbon. 

Water-Gas   Tar.    Water-gas  tar  is  produced  in  the  manufac- 


HIGHWAY  CONSTRUCTION  105 

ture  of  carbureted  water  gas  for  illuminating  purposes  and  results 
from  the  petroleum  product  employed  for  carbureting.  It  is  a 
thin  oily  liquid  containing  a  large  percentage  of  water.  It  is  pre- 
pared for  road  use  by  mechanical  dehydration  and  distillation. 
It  has  a  strong  gassy  odor  when  applied  to  the  road,  but  this  dis- 
appears in  a  few  days. 

Asphaltic  Petroleums.  Asphaltic  petroleums  are  native  petro- 
leums which  yield  asphalts  upon  reduction.  They  are  used  in  the 
crude  state,  or  after  the  illuminating  and  other  oil  constituents 
have  been  removed  by  cracking  or  blowing. 

Asphalt.  Asphalt  is  the  name  by  which  the  native  semi-solid 
and  solid  bitumen  is  known.  Asphalt  is  the  most  permanent  type 
of  binder  and  has  been  used  for  many  years  in  the  construction  of 
sheet-asphalt  pavements.  The  semi-solid  or  tarry  varieties  are 
called  "malthas"  and  are  the  ones  generally  employed  as  a  binder 
for  broken-stone  roads;  the  solid  variety  is  used  almost  exclusively 
for  street  pavements.  Rock  asphalt  or  bituminous  rock  is  the  name 
given  to  a  great  variety  of  sandstone  rocks  more  or  less  saturated 
or  cemented  by  maltha  or  hard  asphalt.  In  referring  to  rock  asphalts 
it  is  customary  to  add  the  name  of  the  locality  where  they  are  found, 
as  "Kentucky  rock  asphalt",  "Italian  rock  asphalt",  etc.  The 
semi-solid  varieties  are  used  in  the  natural  state  or  after  the  water 
and  volatile  hydrocarbons  have  been  removed  by  heating.  The 
hard  varieties  are  prepared  by  softening  with  a  suitable  flux. 

Combinations  of  Asphalt  and  Distillation  Residues.  Com- 
binations of  asphalt  and  the  residue  from  the  distillation  of  tars 
and  petroleums  are  made  by  adding  either  a  refined  maltha  or  a 
pulverized  solid  asphalt;  the  mixing  being  accomplished  by  the 
injection  of  compressed  air  through  suitably  formed  nozzles. 

Tests  for  Bituminous  Materials.  The  bituminous  materials 
are  subjected  to  certain  tests  for  the.  purpose  of  ascertaining  their 
chemical  and  physical  properties.  The  results  of  the  tests  are 
used  in  specifications  to  secure  the  furnishing  of  the  desired  quality 
of  material  and  to  control  the  processes  of  manufacture;  also  to 
form  a  record  by  which  the  behavior  of  the  materials  under  similar 
and  dissimilar  service  conditions  can  be  compared.  The  complex 
character  of  the  materials  requires  a  suitably  equipped  laboratory 
for  the  application  of  the  tests. 


106  HIGHWAY  CONSTRUCTION 

The  tests  are  determinations  of  (1)  amount  of  water  contained; 

(2)  materials  soluble  in  water;  (3)  homogeneity  at  a  temperature 
of  77°   Fahrenheit;    (4)   specific  gravity;    (5)    consistency  or  vis- 
cosity measured  by  a  standard  penetration  machine;  (6)  ductility, 
or  the  distance  the  material  can  be  drawn  out  before  breaking; 
(7)  toughness,  or  resistance  to  fracture  under  blows  in  an  impact 
machine ;  (8)  melting  or  softening  point,  measured  by  a  thermometer 
while  the  temperature  is  raised  through  a  water  or  oil  bath;  (9) 
distillation — the  products  yielded  at  different  temperatures  during 
continuous  distillation  in  a  suitable  flask  or  retort  are  caught  and 
weighed;  (10)  amount  of  free  carbon — a  sample  is  dissolved  in  carbon 
bisulphide,  the  solution  filtered,  and  the  insoluble  residue  weighed; 

(11)  amount  of  fixed  carbon — a  sample  is  placed  in  a  platinum 
crucible  and  heated  until  the  emission  of  flame  and  smoke  ceases, 
then  is  allowed  to  cool  and  the  residue  is  weighed;  and  the  difference 
between  the  weight  of  the  sample  and  the  residue  is  the  fixed  carbon; 

(12)  paraffine — the  presence  of  paraffine  is  determined  by  treating 
a  sample  with  absolute  ether,  freezing  the  mixture,  filtering  the 
precipitate,  evaporating  and  weighing  the  residue;   (13)   amount 
of  bitumen  contained — found  by  weighing  a  sample  of  the  dried 
material,  by  adding  carbon  bisulphide  to  dissolve  the   bitumen, 
and  by  drying  and  weighing  the  residue  after  the  extraction  is 
completed.      The    loss    is    the    amount    of    bitumen    soluble    in 
carbon  bisulphide.     A   sample  also  is  treated  with  naphtha  and 
the  character  of  the  residue  is  noted  as  to  whether  it  is  sticky 
or  oily. 

Concrete  Pavements 

Construction  Methods.  Several  methods  with  many  varia- 
tions are  employed  for  the  construction  of  concrete  pavements. 
The  principal  ones  are:  (1)  grouting  method,  the  construction 
being  commonly  called  "concrete  macadam";  (2)  mixing  method; 

(3)  block  or  cube  method. 

Grouting  Method.  In  this  method  the  stone  is  spread  upon 
the  foundation  and  lightly  rolled,  after  which  a  mixture  of  one 
part  of  cement  and  three  parts  of  sand  in  the  dry  state  is  spread  over 
the  stone  and  swept  into  the  interstices  by  brooms,  then  sprinkled 
with  water  and  rolled;  more  cement  and  sand  are  spread,  sprinkled, 
and  rolled;  the  operation  is  repeated  until  the  interstices  are  filled. 


HIGHWAY  CONSTRUCTION  107 

A  variation  of  this  method,  known  as  the  Hassam  paving,  is  made  " 
by  spreading  the  stone,  ranging  in  size  from  1|  inches  to  2J  inches, 
and  rolling  it  to  a  thickness  of  4  inches,  then  filling  the  interstices 
with  a  grout  composed  of  one  part  cement  and  three  parts  of  sand 
mixed  with  water  in  a  mixing  machine,  from  which  it  flows  over 
the  surface,  the  machines  being  drawn  along  the  roadway  for  this 
purpose;  rolling  is  proceeded  with  at  the  same  time  and  sufficient 
grout  is  applied  to  fill  the  interstices.  On  the  foundation  so  prepared 
a  wearing  surface  is  formed;  the  stone  is  spread  in  the  same  manner 
as  in  the  first  course;  the  grout,  composed  of  one  part  cement  and 
two  parts  sand,  mixed  with  sufficient  water  to  make  it  very  fluid,  is 
applied  by  flowing  over  the  surface  of  the  compacted  stone.  The 
surface  is  finished  by  applying  a  thick  grout  composed  of  one  part 
each  cement,  sand,  and  pea-sized  trap  rock. 

Mixing  Method.  In  this  method  the  ingredients  are  com- 
bined into  concrete  by  either  hand  or  machine  mixing;  the  con- 
crete is  deposited  in  place  in  one  or  two  courses,  the  former  being 
called  "one-course"  pavement  and  the  latter  "two-course"  pave- 
ment. In  the  one-course  method,  the  concrete  mixed  in  the  pro- 
portions of  one  part  cement,  one  and  one-half  parts  sand,  and  three 
parts  stone  is  spread  upon  the  prepared  natural  soil  foundation 
and  tamped  to  a  thickness  of  about  6  inches. 

In  the  two-course  work  the  concrete  mixed  in  the  proportions 
of  one  part  cement,  two  and  one-half  parts  sand,  and  five  parts 
stone  is  spread  upon  the  prepared  natural-soil  foundation  and  com- 
pacted by  rolling  or  tamping  to  the  required  thickness.  On  its 
surface,  and  before  the  cement  has  set,  the  wearing  surface  of  about 
2  inches  in  thickness  is  placed  and  tamped  to  the  required  contour. 
The  mixtures  used  for  the  wearing  surface  vary,  being  composed 
of  sand  and  cement,  or  of  sand,  cement,  and  small  broken  stone. 
The  wearing  surface  of  the  Blome  pavement  is  composed  of  one 
part  cement  and  one  and  one-half  parts  of  aggregate,  which  is 
made  up  of  50  per  cent  J-inch,  30  per  cent  J-inch,  and  20  per  cent 
-tV-inch  granite  screenings.  The  surface  is  formed  into  4J-inch 
by  9-inch  blocks  by  cutting  grooves  \  inch  wide  and  \  inch  deep 
by  means  of  special  tools. 

Materials.  The  materials  used  in  the  construction  of  concrete 
pavements  should  be  selected  with  care.  The  stone  should  be  a 


108  HIGHWAY  CONSTRUCTION 

hard  tough  rock,  free  from  dust  and  dirt,  and  graded  so  as  to  reduce 
voids  to  the  minimum.  The  sand  should  be  free  from  loam,  clay, 
vegetable  and  organic  matter,  and  should  grade  from  coarse  to 
fine.  The  cement  should  be  of  a  quality  to  meet  the  standard 
tests.  The  water  should  be  clean  and  free  from  organic  matter, 
alkalies,  and  acids.  Rapid  drying  of  the  concrete  should  be  pre- 
vented by  covering  it  with  a  canvas  which  is  kept  moistened  with 
water  for  several  hours;  after  its  removal  the  surface  should  be 
covered  with  sand  or  earth  which  is  to  be  kept  moist  for  a  period 
of  two  weeks.  Improperly  mixed  or  constructed  concrete  pave- 
ment will  wear  unevenly,  crack,  and  rapidly  become  very  defective. 

Expansion  Joints.  To  provide  for  the  expansion  and  con- 
traction of  the  concrete  under  changes  of  temperature,  expansion 
joints  are  formed  at  intervals  ranging  from  15  to  50  feet.  The 
edges  of  the  joints  are  protected  from  injury  by  angle  irons,  and 
the  space  between  them,  about  J  inch,  is  filled  with  a  bituminous 
cement  which  extends  the  full  depth  of  the  concrete.  When  the 
concrete  is  laid  between  curbs  longitudinal  joints  from  f  inch  to 
IJ  inches  wide,  filled  with  bituminous  cement,  are  formed  along 
the  curb. 

Reinforced-Concrete  Pavement.  Concrete  pavements  rein- 
forced with  steel  in  the  form  of  woven-wire,  Fig.  70,  expanded  metal, 
and  round  bars  are  constructed  in  two  courses,  the  reinforcement 
being  placed  between  the  foundation  course  and  the  wearing  surface. 

Concrete  with  Bituminous  Surface.  In  this  type  the  surface 
of  the  concrete  pavement,  constructed  by  either  the  grouting  or 
mixing  method,  is  covered  with  a  bituminous  cement  made  from 
either  asphalt,  coal  tar,  or  a  mixture  of  both. 

Block  or  Cube  Pavement.  In'  this  type  of  pavement,  blocks 
or  cubes  of  concrete  are  molded  in  a  machine  or  cast  in  molds. 
The  blocks  are  stacked  and  allowed  to  season  for  three  months, 
during  which  time  they  are  wet  twice  a  day.  They  are  laid  by 
hand  on  a  sand  cushion  spread  upon  the  foundation,  then  are  brought 
to  a  firm  bearing  and  uniform  surface  by  rolling  with  a  light  roller. 
The  surface  is  covered  with  a  layer  of  sand  or  sandy  loam  which 
is  broomed  and  flushed  by  water  into  the  joints  and  the  rolling  is 
repeated;  after  which  the  surface  is  covered  with  a  layer  of  sand, 
and  the  traffic  then  admitted. 


HIGHWAY  CONSTRUCTION 


109 


A  variation  from  the  methods  described  is  made  in  the  patented 
pavement  "rocmac".  This  is  composed  of  broken  stone  cemented 
by  silicate  of  lime,  obtained  by  mixing  powdered  carbonate  of  lime 
with  a  solution  of  silicate  of  soda  and  sugar.  The  silicate  of  lime 
mortar  is  spread  upon  the  foundation  to  a  depth  of  about  2  inches, 
over  which  the  broken  stone  is  distributed  to  such  a  depth  as  will 
give,  when  compacted,  a  depth  of  about  4  inches.  It  is  rolled  and 
sprinkled  with  water  until  the  mortar  flushes  to  the  surface,  and 


Fig.  70.     Laying  Reinforced  Concrete  Road.     Woven  Wire  Fabric  in  Foreground  Ready 

to  Be  Placed  between  Upper  and  Lower  Coat 
Courtesy  of  Municipal  Engineering  and  Contracting  Company,  Chicago 

then  is  covered  with  a  layer  of  stone  screenings  and  finally  opened 
to  traffic. 

MAINTENANCE  AND  IMPROVEMENT  OF  ROADS 

Repair  and  Maintenance  of  Broken=Stone  Roads.  These 
terms  frequently  but  erroneously  are  used  interchangeably.  Repair 
means  the  restoring  of  a  surface  so  badly  worn  that  it  cannot  be 
maintained  in  good  condition.  A  well-maintained  road  should  not 
require  repairs  for  a  considerable  length  of  time.  The  maintenance 
of  a  road  is  the  keeping  of  it,  as  nearly  as  practicable,  in  the  same 
condition  as  it  was  when  constructed. 

Good  maintenance  comprises:  (1)  constant  daily  attention 
to  repair  the  ravages  of  traffic  and  the  elements;  (2)  cleansing  to 


110  HIGHWAY  CONSTRUCTION 

remove  the  detritus  caused  by  wear,  the  horse  droppings,  and 
other  refuse;  and  (3)  application  of  water  or  other  dust  layer. 

When  the  surface  of  a  water-bound  broken-stone  road  requires 
to  be  renewed,  it  is  loosened  and  broken  up  by  scarifying,  the  new 
stone  spread,  rolled,  watered,  and  bound  in  the  same  manner  as  in 
new  construction;  or  the  old  surface  is  cleansed  from  dust  and 
other  matter  by  sweeping  and  washing,  and  the  new  stone  spread 
upon  it,  compacted  and  finished  as  in  new  construction. 

The  resurfacing  of  water-bound  roads  with  a  bituminous  con- 
struction is  becoming  common.  The  methods  employed  are  the 
same  as  heretofore  described  under  "bituminous  macadam". 

Systems  of  Maintenance.  Several  systems  for  maintaining 
roads  are  in  use,  the  one  yielding  the  best  results  being  that  which 
provides  for  the  continuous  employment  of  skilled  workmen.  The 
men  so  employed  become  familiar  with  the  peculiarities  of  the 
sections  in  their  charge  and  with  the  best  way  to  deal  with  them. 
Efficient  maintenance  requires  that  the  surfaces  be  kept  smooth 
so  that  surface  water  may  flow  away  rapidly"  and  that  the  injury 
caused  by  traffic  on  uneven  surfaces  may  be  avoided;  that  incipient 
ruts,  hollows,  and  depressions  be  eliminated  by  cutting  out  the  area 
involved  in  the  form  of  a  square  or  rectangle  and  filling  with  new 
material;  that  dust  and  horse  droppings  be  removed;  that  loose 
stones  be  removed;  that  gutters  be  clear  so  the  rain  water  may  be 
removed  quickly;  that  ditches  and  culverts  be  cleaned  out  in  advance 
of  the  spring  and  fall  rains;  that  weeds  and  grass  be  removed  from 
the  earth  shoulders,  and  that  these  and  the  dust  sweepings  be  not 
left  on  the  sides  of  the  road  to  be  redistributed,  but  be  removed 
immediately  and  disposed  of  in  such  manner  as  will  not  cause  injury; 
that  bridges  be  examined  and  repaired  at  least  twice  a  year. 

Improvement  of  Existing  Roads.  The  improvement  of  existing 
roads  may  be  divided  into  three  branches:  (1)  rectification  of  align- 
ment; (2)  drainage;  (3)  improvement  of  the  surface. 

The  first  of  these  consists  in  the  application  of  the  principles 
which  have  been  laid  down  for  the  location,  etc.,  of  new  roads  and 
will  include  straightening  the  course  by  eliminating  unnecessary 
curves  and  bends ;  improving  the  grade  either  by  avoiding  or  cutting 
down  hills  and  by  embanking  valleys;  increasing  the  width  where 
necessary,  and  rendering  it  uniform  throughout. 


HIGHWAY  CONSTRUCTION  111 

The  second,  or  drainage,  consists  in  applying  the  principles  laid 
down  for  the  drainage  of  new  roads,  and  in  constructing  the  works 
necessary  to  give  them  effect. 

The  third,  or  improvement  of  the  surface,  consists  in  improving 
the  surface  by  any  of  the  methods  previously  described  and  that 
the  funds  available  will  permit. 

Value  of  Improvement.  The  improvement  of  roads  is  chiefly  an 
economical  question  relating  to  the  wTaste  of  effort  and  to  the  saving 
of  expenditure.  Good  roads  reduce  the  resistance  to  locomotion, 
and  this  means  reduction  of  the  effort  required  to  move  a  given  load. 
Any  effort  costs  something,  and  so  the  smallest  effort  costs  the  least, 
and  therefore  the  smoothest  road  saves  the  most  money  for  every- 
one who  traverses  it  with  a  vehicle. 

Cost  of  Improvement.  Before  undertaking  any  improvement 
generally  it  is  required  to  know  the  cost  of  the  proposed  improve- 
ment and  the  benefits  it  will  produce.  In  the  improvement  of  roads 
the  amount  of  money  that  may  be  expended  profitably  for  any 
proposed  improvement  may  be  calculated  with  sufficient  accuracy 
by  obtaining  first  the  following  data:  (1)  the  quantity  and  quality 
of  the  traffic  using  the  road;  (2)  the  cost  of  haulage;  (3)  plan  and 
profile  of  the  road;  and  (4)  character  and  cost  of  the  proposed 
improvements.  From  the  data  ascertain  the  total  annual  traffic  and 
the  total  annual  cost  of  hauling  it.  Next,  calculate  the  annual  cost 
of  hauling  the  given  tonnage  over  the  road  when  improved.  Then 
the  difference  between  the  two  costs  will  represent  the  annual  inter- 
est on  the  sum  that  may  be  expended  in  making  the  improvement. 
For  example,  if  the  annual  cost  of  haulage  over  the  existing  road  is 
$10,000  and  the  cost  for  hauling  the  same  tonnage  over  the  improved 
road  will  be  $7000,  the  difference,  $3000,  with  money  at  6  per  cent 
per  annum,  represents  the  sum  of  $50,000  that  logically  may  be 
appropriated  to  carry  out  the  improvement. 

Traffic  Census.  The  direction,  character,  and  amount  of  traffic 
using  a  road  is  obtained  by  direct  observation  during  different 
seasons  of  the  year.  As  a  preliminary  to  observing  the  traffic  it  is 
usual  to  determine  the  weight  of  the  vehicles;  this  is  done  by  weigh- 
ing typical  vehicles  and  by  establishing  an  average  weight  for  each 
type.  The  traffic  is  classified  according  to  the  motive  power — as 
horse-drawn  vehicles  and  motor  vehicles.  Each  of  these  classes  is 


112 


HIGHWAY  CONSTRUCTION 


TABLE  XI 

Traffic  Census 

Average  Hours  per  Day ;  for 

Taken  at 
By 


.Days 


EMPTY  VEHICLES 

LOADED  VEHICLES 

Nov.  to 
March 

Aug.  to 
Oct. 

Nov.  to 
March 

Aug.  to 
Oct. 

Pleasure 

Horse  <                        /T  -  ,  , 

[Light 

[  Commercial  <  Medium 
(Heavy 

fMotorcycles 
Pleasure  j  Runabouts 
[Touring  cars 
Motor  • 
(Light 
Commercial  j  Medium 
(  Heavy 

divided  into  pleasure  and  commercial  traffic,  the  latter  class  being 
subdivided  into  loaded  and  non-loaded  vehicles.  The  number  of 
horses  to  a  vehicle  in  horse-drawn  traffic  and  the  speed  of  motor 
vehicles  may  be  noted.  A  summary  of  data  is  suggested  in  Table  XL 

The  observations  are  made  from  6  a.  m.  to  6  p.  m.,  during  a  period 
of  seven  days  each  month,  with  occasional  observations,  from  6  p.  m. 
to  6  a.  m.  or  for  the  entire  24  hours  if  the  amount  of  traffic  requires  it. 

The  weight  of  the  traffic  is  ascertained  by  multiplying  the 
number  of  each  kind  of  vehicle  by  the  average  weight  established 
for  that  type. 


HIGHWAY  CONSTRUCTION 

PART  II 


CITY  STREETS  AND  HIGHWAYS 

The  first  work  requiring  the  skill  of  the  engineer  is  the  laying 
out  of  town  sites  properly,  especially  with  reference  to  the  future 
requirements  of  a  large  city,  where  any  such  possibility  exists. 
Few  if  any  of  our  large  cities  were  so  planned.  The  same  principles, 
to  a  limited  extent,  are  applicable  to  all  towns  or  cities.  The  topog- 
raphy of  the  site  should  be  studied  carefully,  and  the  street  lines 
adapted  to  it.  These  lines  should  be  laid  out  systematically,  with  a 
view  to  convenience  and  comfort,  and  also  with  reference  to  econ- 
omy of  construction,  future  sanitary  improvements,  grades,  and 
drainage. 

Arrangement  of  City  Streets.  Generally,  the  best  method  of 
laying  out  streets  is  in  straight  lines,  with  frequent  and  regular  inter- 
secting streets,  especially  for  the  business  parts  of  a  city.  When 
there  is  some  centrally  located  structure,  such  as  a  courthouse,  city 
hall,  market,  or  other  prominent  building,  it  is  very  desirable  to  have 
several  diagonal  streets  leading  thereto.  In  the  residence  portions 
of  cities,  especially  if  on  hilly  ground,  curves  may  replace  straight 
lines  with  advantage,  by  affording  better  grades  at  less  cost  of  grad- 
ing, and  by  improving  property  through  avoiding  heavy  embank- 
ments or  cuttings. 

Width  of  Streets.  The  width  of  streets  should  be  proportioned 
to  the  character  of  the  traffic  that  will  use  them.  No  rule  can  be 
laid  down  by  which  to  determine  the  best  width  of  streets;  but  it 
may  be  said  safely  that  a  street  which  is  likely  to  become  a  com- 
mercial thoroughfare  should  have  a  width  of  not  less  than  120  feet 
between  the  building  lines — the  carriage-way  80  feet  wide,  and  the 
sidewalks  each  20  feet  wide. 

In  streets  occupied  entirely  by  residences  a  carriage-way  32  feet 
wide  will  be  ample,  but  the  width  between  the  building  lines  may  be 


114 


HIGHWAY  CONSTRUCTION 


as  great  as  desired.  The  sidewalks  may  be  any  amount  over  10  feet 
which  fancy  dictates.  Whatever  width  is  adopted  for  them,  not 
more  of  it  than  8  feet  need  be  paved,  the  remainder  being  occupied 
with  grass  and  trees. 

Street  Grades.  The  grades  of  city  streets  depend  upon  the 
topography  of  the  site.  The  necessity  of  avoiding  deep  cuttings  or 
high  embankments  which  seriously  would  affect  the  value  of  adjoin- 
ing property  for  building  purposes,  often  demands  steeper  grades 
than  are  permissible  on  country  roads.  Many  cities  have  paved 
streets  on  20  per  cent  grades.  In  establishing  grades  through  unim- 
proved property,  they  usually  may  be  laid  with  reference  to  securing 
the  most  desirable  percentage  within  a  proper  limit  of  cost.  But 

when  improvements  already 
have  been  made  and  have  been 
located  with  reference  to  the 
natural  surface  of  the  ground, 
the  matter  of  giving  a  desirable 
grade  without  injury  to  adjoin- 
ing property  frequently  is  one 
of  extreme  difficulty.  In  such 
cases  it  becomes  a  question  of 


a 


Fig.  71.     Diagram    Showing    Arrangement    of 
Grades   at   Street    Intersections 


how  far  individual  interests  shall 
be  sacrificed  to  the  general  good. 
There  are,  however,  certain  con- 
ditions which  it  is  important  to 
bear  in  mind:  (1)  That  the 
longitudinal  crown  level  should  be  sustained  uniformly  from  street 
to  street  intersection,  whenever  practicable.  (2)  That  the  grade 
should  be  sufficient  to  drain  the  surface.  (3)  That  the  crown  levels 
at  all  intersections  should  be  extended  transversely,  to  avoid  forming 
a  depression  at  the  junction. 

A  rrangements  of  Grades  at  Street  Intersections .  The  best  arrange- 
ment for  intersections  of  streets  when  either  or  both  have  much 
inclination  is  a  matter  which  requires  careful  consideration  and 
upon  which  much  diversity  of  opinion  exists.  No  hard  or  fast  rule 
can  be  laid  down;  each  will  require  special  adjustment.  The  best 
and  simplest  method  is  to  make  level  the  rectangular  space  aaaaaaaa, 
Fig.  71,  with  a  rise  of  one-half  inch  in  10  feet  from  AAAA  to  B, 


HIGHWAY  CONSTRUCTION 


115 


placing  gulleys  at  AAAA  and  the  catch  basins  at  ccc.  When  this 
method  is  not  practicable,  adopt  such  a  grade  (but  one  not  exceeding 
2J  per  cent)  that  the  rectangle  AAAA  shall  appear  to  be  nearly 
level;  but  to  secure  this  it  must  have  actually  a  considerable  dip  in 
the  direction  of  the  slope  of  the  street.  If  steep  grades  are  con- 
tinued across  intersections,  they  introduce  side  slopes  in  the  streets 
thus  crossed,  which  are  troublesome,  if  not  dangerous,  to  vehicles 
turning  the  corners,  especially  the  upper  ones.  Such  intersections 


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Fig.  72.     Diagram  Showing  Arrangement  of  Intersections  for  Steep  Grades  in 
Duluth,  Minnesota 

are  especially  objectionable  in  rainy  weather.  The  storm  water 
will  fall  to  the  lowest  point,  concentrating  a  large  quantity  of  water 
at  two  receiving  basins,  which,  with  a  broken  grade,  could  be  divided 
among  four  or  more  basins. 

Fig.  72  shows  the  arrangement  of  intersections  in  steep  grades 
adapted  for  the  streets  of  Duluth,  Minnesota.  From  this  it  will  be 
seen  that  at  these  intersections  the  grades  are  flattened  to  3  per 
cent  for  the  width  of  the  roadway  of  the  intersecting  streets,  and  that 
the  grade  of  the  curbs  is  flattened  to  8  per  cent  for  the  width  of  the 


116 


HIGHWAY  CONSTRUCTION 


intersecting  sidewalks.  Grades  of  less  amount  on  roadway  or  side- 
walk are  continuous.  The  elevation  of  block  corners  is  found  by 
adding  together  the  curb  elevations  at  the  faces  of  the  block  corners, 
and  2J  per  cent  of  the  sum  of  the  widths  of  the  two  sidewalks  at  the 
corner,  and  dividing  the  whole  by  two.  This  gives  an  elevation 
equal  to  the  average  elevation  of  the  curbs  at  the  corners,  plus  an 
average  rise  of  2J  per  cent  across  the  width  of  the  sidewalk. 

"Accommodation  summits"  have  to  be  introduced  between 
street  intersections  in  two  general  cases:  (1)  in  hilly  localities,  to 
avoid  excessive  excavation;  and  (2)  when  the  intersecting  streets 
are  level  or  nearly  so,  for  the  purpose  of  obtaining  the  fall  necessary 
for  surface  drainage. 

The  elevation  and  location  of  such  a  summit  may  be  calculated 
as  follows:  Let  A,  Fig.  73,  be  the  elevation  of  the  highest  corner; 


Fig.  73.     Diagrams  for  Calculating  "Accommodation  Summits"  between 
Street   Intersections 


B,  the  elevation  of  the  lowest  corner;  D,  the  distance  from  corner 
to  corner;  and  R,  the  rate  of  the  accommodation  grade.  The 
elevation  of  the  summit  is  equal  to 

DXR+A+B 


The  distance  from  A  or  B  is  found  by  subtracting  the  elevation  of 
either  A  or  B  from  this  quotient,  and  dividing  the  result  by  the  rate 
of  grade.  Or  the  summit  may  be  located  mechanically  by  specially 
prepared  scales.  Prepare  two  scales  divided  to  correspond  to  the 
rate  of  grade;  that  is,  if  the  rate  of  grade  be  1  foot  per  100  feet,  then 
one  division  of  the  scale  should  equal  100  feet  on  the  map  scale. 


HIGHWAY  CONSTRUCTION  117 

These  divisions  may  be  subdivided  into  tenths.  One  scale  should 
read  from  right  to  left,  and  one  from  left  to  right. 

To  use  the  scales,  place  them  on  the  map  so  that  their  figures 
correspond  with  the  corner  elevations;  then,  as  the  scales  read  in 
opposite  directions,  there  is  of  course  some  point  at  which  the  oppo- 
site readings  will  be  the  same.  This  point  is  the  location  of  the 
summit,  and  the  figures  read  off  the  scale  give  its  elevation.  If  the 
difference  in  elevation  of  the  corners  is  such  as  not  to  require  an 
intermediate  summit  for  drainage,  it  will  be  apparent  as  soon  as 
the  scales  are  placed  in  position. 

When  an  accommodation  summit  is  employed,  it  should  be 
formed  by  joining  the  two  straight  grade  lines  by  a  vertical  curve, 
as  described  in  Part  I.  The  curve  should  be  used  both  in  the  crown 
of  the  street  and  in  the  curb  and  footpath. 

Where  the  grade  is  level  between  intersections,  sufficient  fall  for 
surface  drainage  may  be  secured  without  the  aid  of  accommodation 
summits,  by  arranging  the  grades  as  shown  in  Fig.  74.  The  curb  is 

Curb Level 


_.  ~  •—  —  '  Bottom  of  Gutter  — , . 

Fig.  74.     Diagram  Showing  Arrangement  of  Grades  to  Avoid 
"Accommodation  Summits" 

set  level  between  the  corners;  a  summit  is  formed  in  the  gutter;  and 
receiving  basins  are  placed  at  each  corner. 

Transverse  Grade.     In  its  transverse  grade  the  street  should  be 
level ;  that  is,  the  curbs  on  opposite  sides  should  be  at  the  same  level, 


Fig.  75.     Street  with  Unequal  Transverse  Grade  but  with  Level  Street 

and  the  street  crown  rise  equally  from  each  side  to  the  center.  But 
in  hillside  streets  this  condition  cannot  be  fulfilled  always,  and 
opposite  sides  of  the  street  may  differ  as  much  as  5  feet.  In  such 
cases  the  engineer  will  have  to  use  his  discretion  as  to  whether  he 
shall  adopt  a  straight  slope  inclining  to  the  lower  side,  thus  draining 


118 


HIGHWAY  CONSTRUCTION 


the  whole  street  by  the  lower  gutter,  or  adopt  the  three-curb  method 
and  sod  the  slope  of  the  higher  side. 

In  the  improvement  of  old  streets  with  the  sides  at  different 
levels,  much  difficulty  will  be  met,  especially  where  shade  trees  have 

1 


Fig.  76.     Street  with  Unequal  Transverse  Grade  Inclined  so  as  to  Drain  by  Lower  Gutter 

to  be  spared.  In  such  cases,  recognized  methods  have  to  be  aban- 
doned, and  the  engineer  will  have  to  adopt  methods  of  overcoming 
the  difficulties  in  accordance  with  the  conditions  and  necessities  of 
each  particular  case.  Figs.  75,  76,  and  77  illustrate  several  typical 


Fig.  77.     Street  with  Unequal  Transverse  Grade  with  Three  Curbs  and  Higher  Slope  Sodded 

arrangements  in  the  cases  of  streets  where  the  opposite  sides  are  at 
different  levels. 

Transverse  Contour  or  Crown.  The  reason  for  crowning  a 
pavement — i.e.,  making  the  center  higher  than  the  sides — is  to 
provide  for  the  rapid  drainage  of  the  surface.  The  most  suitable 
form  for  the  crown  is  the  parabolic  curve,  which  may  be  started  at 
the  curb  line,  or  at  the  edge  of  the  gutter  adjoining  the  carriage-way, 


Fig.  78.     Method  of  Obtaining  Transverse  Contour  or  Crown  of  a  Road 

about  one  foot  from  the  curb.  Fig.  78  shows  this  form,  which  is 
obtained  by  dividing  the  abscissa,  or  width  from  the  center  of  the 
street  to  the  gutter,  into  ten  equal  parts,  and  by  dropping  perpen- 
diculars at  each  of  these  divisions,  the  lengths  of  which  are  deter- 
mined by  multiplying  the  rise  at  the  center  by  the  square  of  the 


HIGHWAY  CONSTRUCTION 


119 


TABLE  XII 

Rise  of  Pavement  Center  above  Gutter  for 
Different  Paving  Materials 


PAVING  MATERIAL 

PROPORTIONS  OF  RISE  AT 
CENTER  TO  WIDTH  OF 
CARRIAGE-WAY 

Wood  blocks 
Stone  blocks 
Brick 
Asphalt 

1  :  100 
1  :80 
1  :80 
1  :80 

successive  values  of  the  abscissas.  The  amounts  thus  obtained  can 
be  added  to  the  rod  readings;  and  the  stakes,  set  at  the  proper  dis- 
tance across  the  street,  with  their  tops  at  this  level,  will  give  the 
required  curve. 

The  amount  of  transverse  rise,  or  the  height  of  the  center  above 
the  gutters,  varies  with  the  different  paving  materials;  smooth  pave- 
ments requiring  the  least,  and  rough  ones  and  earth  the  greatest 
rise.  The  rise  is  generally  stated  in  a  proportion  of  the  width  of  the 
carriage-way.  The  most  suitable  proportions  are  shown  in  Table  XII. 

Drainage  of  Streets.  Sub-Foundation  Drainage.  The  sub- 
foundation  drainage  of  streets  cannot  be  effected  by  transverse 
drains,  because  of  their  liability  to  disturbance  by  the  introduction 
of  gas,  water,  and  other  pipes. 

Longitudinal  drains  must  be  depended  upon  entirely;  they  may 
be  constructed  of  the  same  materials  and  in  the  same  manner  as  road 
drains.  The  number  of  these  longitudinal  drains  must  depend  upon 
the  character  of  the  soil.  If  the  soil  is  moderately  retentive,  a  single 
row  of  tiles  or  a  hollow  invert  placed  under  the  sewer  in  the  center  of 
the  street  generally  will  be  sufficient;  or  two  rows  of  tiles  may  be 
employed,  one  placed  outside  each  curb  line.  If,  on  the  other  hand, 
the  soil  is  exceedingly  wet  and  the  street  very  wide,  four  or  more 
lines  may  be  employed.  These  drains  may  be  permitted  to  dis- 
charge into  the  sewers  of  the  transverse  streets. 

Surface  Drainage.  The  removal  of  water  falling  on  the  street 
surface  is  provided  for  by  collecting  it  in  the  gutters,  from  which  it  is 
discharged  into  the  sewers  or  other  channels  by  means  of  catch  basins 
placed  at  all  street  intersections  and  dips  in  the  street  grades. 

Gutters.  The  gutters  must  be  of  sufficient  depth  to  retain  all  the 
water  which  reaches  them  and  prevent  its  overflowing  on  the  foot- 


120  HIGHWAY  CONSTRUCTION 

path.     The  depth  should  never  be  less  than  6  inches,  and  very 
rarely  need  be  more  than  10  inches. 

Catch  Basins.  Catch  basins  are  of  various  forms,  usually  circu- 
lar or  rectangular,  built  of  brick  masonry  coated  with  a  plaster  of 
Portland  cement.  Whichever  form  is  adopted,  they  should  fulfill 
the  following  conditions : 

(1)  The  inlet  and  outlet  should  have  sufficient  capacity  to 
receive  and  discharge  all  water  reaching  the  basin. 

(2)  The  basins  should  have  sufficient  capacity  below  the  outlet 
to  retain  all  sand  and  road  detritus,  and  prevent  its  being  carried 
into  the  sewer. 

(3)  They  should  be  trapped  so  as  to  prevent  the  escape  of 
sewer  gas.     (This  requirement  frequently  is  omitted,  to  the  detri- 
ment of  the  health  of  the  people.) 

(4)  They  should  be  constructed  so  that  the  pit  can  be  cleaned 
out  easily. 

(5)  The  inlet  should  be  constructed  so  as  not  to  be  choked 
easily  by  leaves  or  debris. 

(6)  They  must  offer  the  least  possible  obstruction  to  traffic. 

(7)  The  pipe  connecting  the  basin  to  the  sewer  should  be 
freed  easily  of  any  obstruction. 

The  bottoms  of  the  basins  should  be  6  or  8  feet  below  the  street 
level;  and  the  water  level  in  them  should  be  from  3  to  4  feet  lower 
than  the  street  surface,  as  a  protection  against  freezing. 

The  capacity  and  number  of  basins  will  depend  upon  the  area 
of  the  surface  which  they  drain. 

In  streets  having  level  or  light  longitudinal  grades,  gullies  may 
be  formed  along  the  line  of  the  gutter  at  such  intervals  as  may  be 
found  necessary. 

Catch  basins  usually  are  placed  at  the  curb  line.  In  several 
cities,  the  basin  is  placed  in  the  center  of  the  street,  and  connects  to 
inlets  placed  at  the  curb  line.  This  reduces  the  cost  of  construction 
and  cleaning,  and  removes  from  the  sidewalk  the  dirty  operations  of 
cleaning  the  basins. 

Catch  basins  and  gully  pits  require  cleaning  out  at  frequent 
intervals;  otherwise  the  odor  arising  from  the  decomposing  matter 
contained  in  them  will  be  very  offensive.  No  rule  can  be  laid  down 
for  the  intervals  at  which  the  cleaning  should  be  done,  but  they  must 


HIGHWAY  CONSTRUCTION  121 

be  cleaned  often  enough  to  prevent  the  matter  in  them  from  putre- 
fying. There  is  no  uniformity  of  practice  observed  by  cities  in  this 
matter;  in  some,  the  cleaning  is  done  but  once  a  year;  in  others,  after 
every  rain-storm;  in  still  others,  at  intervals  of  three  or  four  months; 
while  in  a  few  cities  the  basins  are  cleaned  out  once  a  month. 

FOUNDATIONS 

The  stability,  permanence,  and  maintenance  of  any  pavement 
depend  upon  its  foundation.  If  the  foundation  is  weak,  the  surface 
soon  will  settle  unequally,  forming  depressions  and  ruts.  With  a 
good  foundation,  the  condition  of  the  surface  will  depend  upon 
the  material  employed  for  the  pavement  and  upon  the  manner  of 
laying  it. 

The  essentials  necessary  to  the  forming  of  a  good  foundation  are : 

(1)  The  entire  removal  of  all  vegetable,  perishable,  and  yielding 
matter.     It  is  of  no  use  to  lay  good  material  on  a  bad  substratum. 

(2)  The  drainage  of  the  subsoil  wherever  necessary.    A  per- 
manent foundation  can  be  secured  only  by  keeping  the  subsoil  dry; 
for,  where  water  is  allowed  to  pass  into  and  through  it,  its  weak  spots 
will  be  discovered  quickly,  and  settlement  will  take  place. 

(3)  The  thorough  compacting  of  the  natural  soil  by  rolling  with 
a  roller  of  proper  weight  and  shape  until  there  is  formed  a  uniform 
and  unyielding  surface. 

(4)  The  placing  on  the  natural  soil  so  compacted  of  a  thickness 
of  an  impervious  and  incompressible  material  sufficient  to  cut  off  all 
communication  between  the  soil  and  the  bottom  of  the  pavement. 

The  character  of  the  natural  soil  over  which  the  roadway  is  to  be 
built  has  an  important  bearing  upon  the  kind  of  foundation  and  the 
manner  of  forming  it;  each  class  of  soil  will  require  its  own  special 
treatment.  Whatever  its  character,  it  must  be  brought  to  a  dry  and 
tolerably  hard  condition  by  draining  and  rolling.  Sand  and  gravels 
which  do  not  hold  water,  present  no  difficulty  in  securing  a  solid  and 
secure  foundation;  clays  and  soils  retentive  of  water  are  the  most 
difficult.  Clay  should  be  excavated  to  a  depth  of  at  least  8  inches 
below  the  bottom  of  the  finished  covering;  and  the  space  so  excavated 
should  be  filled  in  with  sand,  furnace  slag,  ashes,  coal  dust,  oyster 
shells,  broken  brick,  or  other  materials  which  are  not  absorbent  of 
water  excessively.  A  clay  soil  or  one  retaining  water  may  be  cheaply 


122  HIGHWAY  CONSTRUCTION 

and  effectually  improved  by  laying  cross  drains  with  open  joints  at 
intervals  of  50  or  100  feet.  These  drains  should  be  not  less  than 
18  inches  below  the  surface,  and  the  trenches  should  be  filled  with 
gravel.  They  should  be  4  inches  in  internal  diameter,  and  should 
empty  into  longitudinal  drains. 

Sand  and  planks,  gravel  and  broken  stone  successively  have 
been  used  to  form  the  foundation  for  pavements;  but,  although  emi- 
nently useful  materials,  their  application  to  this  purpose  always  has 
been  a  failure.  Being  inherently  weak  and  possessing  no  cohesion, 
the  main  reliance  for  both  strength  and  wear  must  be  placed  upon 
the  surface  covering.  This  covering — usually  (except  in  case  of  sheet 
asphalt)  composed  of  small  units,  with  joints  between  them  varying 
from  |  inch  to  1|  inches — posesses  no  elements  of  cohesion;  and 
under  the  blows  and  vibrations  of  traffic  the  independent  units  or 
blocks  will  settle  and  be  jarred  loose.  On  account  of  their  porous 
nature,  the  subsoil  quickly  becomes  saturated  with  urine  and  sur- 
face waters,  which  percolate  through  the  joints;  winter  frosts  upheave 
them;  and  the  surface  of  the  street  becomes  blistered  and  broken  up 
in  dozens  of  places. 

Concrete.  As  a  foundation  for  all  classes  of  pavement  (broken 
stone  excepted),  hydraulic-cement  concrete  is  superior  to  any  other. 
When  properly  constituted  and  laid,  it  becomes  a  solid,  coherent  mass, 
capable  of  bearing  great  weight  without  crushing.  If  it  fail  at  all,  it 
must  fail  altogether.  The  concrete  foundation  is  the  most  costly, 
but  this  is  balanced  by  its  permanence  and  by  the  saving  in  the  cost 
of  repairs  to  the  pavement  which  it  supports.  It  admits  of  access  to 
subterranean  pipes  with  less  injury  to  the  neighboring  pavement 
than  any  other,  for  the  concrete  may  be  broken  through  at  any 
point  without  unsettling  the  foundation  for  a  considerable  distance 
around  it,  as  is  the  case  with  sand  or  other  incoherent  material;  and 
when  the  concrete  is  replaced  and  set,  the  covering  may  be  reset  at 
its  proper  level,  without  the  uncertain  allowance  for  settlement 
which  is  necessary  in  other  cases. 

Thickness  of  Course.  The  thickness  of  the  concrete  bed  must 
be  proportioned  by  the  engineer;  it  should  be  sufficient  to  provide 
against  breaking  under  transverse  strain  caused  by  the  settlement  of 
the  subsoil.  On  a  well-drained  soil,  6  inches  will  be  found  sufficient; 
but  in  moist  and  clayey  soils,  12  inches  will  not  be  excessive.  On 


HIGHWAY  CONSTRUCTION  123 

such  soils  a  layer  of  sand  or  gravel,  spread  and  compacted  before 
placing  the  concrete,  will  be  found  very  beneficial. 

The  proportions  of  the  ingredients  required  for  the  manufacture 
of  concrete  are  ascertained  by  measuring  the  voids  in  each  ingredient. 
The  strongest  concrete  will  be  produced  when  the  volume  of  cement 
is  slightly  in  excess  of  that  required  to  fill  the  voids  in  the  sand,  and 
the  volume  of  the  combined  cement  and  sand  exceeds  by  about  10 
per  cent  the  volume  of  the  voids  in  the  stone  or  other  material  used 
for  the  aggregate.  Concrete  frequently  is  mixed  in  the  arbitrary 
proportions  of  1  part  of  cement,  3  parts  of  sand,  and  6  parts  of  stone, 
and  although  the  results  have  been  satisfactory,  the  proportions  may 
not  be  the  most  economical. 

The  ingredients  of  the  concrete  should  be  thoroughly  mixed 
with  just  sufficient  water  to  produce  a  plastic  mass,  without  any 
surplus  water  running  from  it.  After  mixing,  the  concrete  should 
be  deposited  quickly  in  place,  and  brought  to  a  uniform  surface  and 
thickness  by  raking,  then  tamped  until  the  mortar  flushes  to  the 
surface,  then  left  undisturbed  until  set.  The  surface  of  concrete 
laid  during  dry,  warm,  weather  should  be  protected  from  the  drying 
action  of  the  sun  while  the  initial  setting  is  in  progress.  This  may 
be  accomplished  by  sprinkling  with  water  as  frequently  as  the  rate 
of  evaporation  demands  or  by  covering  it  with  a  layer  of  damp 
sand,  straw,  hay,  or  canvas.  During  freezing  weather  it  is  customary 
to  suspend  the  laying  of  concrete  for  the  reason  that  alternate  freez- 
ing and  thawing  disintegrate  it. 

Measuring  Voids  in  the  Stone  and  Sand.  The  simplest  method 
for  measuring  the  voids  and  one  sufficiently  accurate  for  the  manu- 
facture of  concrete  is  the  "pouring  method'5  in  which  a  suitable 
vessel  of  known  capacity  (usually  one  cubic  foot)  is  filled  with  the 
material,  in  which  it  is  desired  to  ascertain  the  voids.  Water  then 
is  poured  into  the  vessel  until  its  surface  is  flush  with  the  surface  of 
the  material.  The  water  is  measured,  and  its  amount  is  considered 
to  equal  the  total  of  the  voids. 

STONE=BLOCK  PAVEMENTS 

Stone  blocks  commonly  are  employed  for  pavements  where 
traffic  is  heavy.  The  material  of  which  the  blocks  are  made  should 
possess  sufficient  hardness  to  resist  the  abrasive  action  of  traffic,  and 


124 


HIGHWAY  CONSTRUCTION 


TABLE  XIII 

Specific  Gravity,  Weight,  Resistance  to  Crushing,  and  Absorptive 
Power  of  Stones 


MATERIAL 

SPECIFIC 
GRAVITY 

WEIGHT 
(Ib.  per 
cu.  ft.) 

RESISTANCE 
TO  CRUSHING 
(Ib.  per  sq.  in.) 

PERCENTAGE 
OF  WATER 
ABSORBED 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Granite 

2.60 

2.80 

163 

176 

12,000 

35,000 

0.066 

0.155 

Trap 

2.86 

3.03 

178 

189 

19,000 

24,000 

0.000 

0.019 

Sandstone 

2.23 

2.75 

137 

170 

5,000 

18,000 

0.410 

5.480 

Limestone 

1.90 

2.75 

118 

175 

7,000 

20,000 

0.200 

5.000 

Brick,  paving 

1.95 

2.55 

10,000 

20,000 

sufficient  toughness  to  prevent  them  from  being  broken  by  the 
impact  of  loaded  wheels.  The  hardest  stones  will  not  give  neces- 
sarily the  best  results  in  the  pavement,  since  a  very  hard  stone 
usually  wears  smooth  and  becomes  slippery.  The  edges  of  the 
block  chip  off,  and  the  upper  face  becomes  rounded,  thus  making  the 
pavement  very  rough. 

The  stone  sometimes  is  tested  to  determine  its  strength,  resist- 
ance to  abrasion,  etc. ;  but,  as  the  conditions  of  use  are  quite  different 
from  those  under  which  it  may  be  tested,  such  tests  are  seldom 
satisfactory.  However,  examination  of  a  stone  as  to  its  structure, 
the  closeness  of  its  grain,  its  homogeneity,  porosity,  etc.,  may  assist 
in  forming  an  idea  of  its  value  for  use  in  a  pavement.  A  low  degree 
of  permeability  usually  indicates  that  the  material  will  not  be 
greatly  affected  by  frost.  For  data  see  Table  XIII. 

Materials.  Granite.  Granite  is  employed  more  extensively  for 
stone-block  paving  than  is  any  other  variety  of  stone;  and  because 
of  this  fact,  the  term  "granite  paving"  is  generally  used  as  being 
synonymous  with  stone-block  paving.  The  granite  employed  should 
be  of  a  tough,  homogeneous  nature.  The  hard,  quartz  granites 
usually  are  brittle,  and  do  not  wear  well  under  the  blows  of  horses' 
feet  or  the  impact  of  vehicles;  granite  containing  a  high  percentage 
of  feldspar  will  be  injuriously  affected  by  atmospheric  changes;  and 
granite  in  which  mica  predominates  will  wear  rapidly  on  account 
of  its  laminated  structure.  Granite  possesses  the  very  important 
property  of  splitting  in  three  planes  at  right  angles  to  one  another, 


HIGHWAY  CONSTRUCTION  125 

so  that  paving  blocks  may  readily  be  formed  with  nearly  plane  faces 
and  square  corners.  This  property  is  called  the  rift  or  cleavage. 

Sandstones.  Sandstones  of  a  close-grained,  compact  nature 
often  give  very  satisfactory  results  under  heavy  traffic.  They  are 
less  hard  than  granite,  and  wear  more  rapidly,  but  do  not  become 
smooth  and  slippery.  Sandstones  are  generally  known  in  the 
market  by  the  name  of  the  quarry  or  place  where  produced  as 
"Medina",  "Berea",  etc. 

Trap  Rock.  Trap  rock,  while  answering  well  the  requirements 
as  to  durability  and  resistance  to  wear,  is  objectionable  on  account 
of  its  tendency  to  wear  smooth  and  become  slippery;  it  is  also  diffi- 
cult to  break  into  regular  shapes. 

Limestone.  Limestone  usually  has  not  been  successfully  em- 
ployed in  the  construction  of  block  pavements,  on  account  of  its 
lack  of  durability  against  atmospheric  influences.  The  action  of 
frost  commonly  splits  the  blocks;  and  traffic  shivers  them,  owing 
to  the  lamination  being  vertical. 

Cobblestone  Pavement.  Cobblestones  bedded  in  sand  possess 
the  merit  of  cheapness,  and  afford  an  excellent  foothold  for  horses; 
but  the  roughness  of  such  pavements  requires  the  expenditure  of  a 
large  amount  of  tractive  energy  to  move  a  load  over  them.  Aside 
from  this,  cobblestones  are  entirely  wanting  in  the  essential  requisites 
of  a  good  pavement.  The  stones  being  of  irregular  size,  it  is  almost 
impossible  to  form  a  bond  or  to  hold  them  in  place.  Under  the 
action  of  the  traffic  and  frost,  the  roadway  soon  becomes  a  mass  of 
loose  stones.  Moreover,  cobblestone  pavements  are  difficult  to 
keep  clean,  and  very  unpleasant  to  travel  over. 

Belgian=Block  Pavement.  Cobblestones  were  displaced  by 
pavements  formed  of  small  cubical  blocks  of  stone.  This  type  of 
pavement  was  laid  first  in  Brussels,  thence  imported  to  Paris,  and 
from  there  taken  to  the  United  States,  where  it  has  been  widely 
known  as  the  "Belgian-block"  pavement.  It  has  been  largely  used 
in  New  York  City,  Brooklyn,  and  neighboring  towns,  the  material 
being  trap  rock  obtained  from  the  Palisades  on  the  Hudson  River. 

The  stones,  being  of  regular  shape,  remain  in  place  better  than 
cobblestones;  but  the  cubical  form  (usually  5  inches  in  each  dimen- 
sion) is  a  mistake.  The  foothold  is  bad;  the  stones  wear  round;  and 
the  number  of  joints  is  so  great  that  ruts  and  hollows  are  quickly 


126  HIGHWAY  CONSTRUCTION 

formed.  This  pavement  offers  less  resistance  to  traction  than  cobble- 
stones, but  it  is  almost  equally  rough  and  noisy. 

Granite=Block  Pavement.  The  Belgian  block  gradually  has 
been  displaced  by  the  introduction  of  rectangular  blocks  of  granite. 
Blocks  of  comparatively  large  dimensions  were  employed  at  first. 
They  were  from  6  to  8  inches  in  width  on  the  surface,  from  10  to  20 
inches  in  length,  with  a  depth  of  9  inches.  They  merely  were  placed 
in  rows  on  the  subsoil,  perfunctorily  rammed,  the  joints  filled  with 
sand,  and  the  street  thrown  open  to  traffic.  The  unequal  settlement 
of  the  blocks,  the  insufficiency  of  the  foothold,  and  the  difficulty  of 
cleansing  the  street,  led  to  the  gradual  development  of  the  latest 
type  of  stone-block  pavement,  which  consists  of  narrow,  rectangular 
blocks  of  -granite,  properly  proportioned,  laid  on  an  unyielding  and 
impervious  foundation,  with  the  joints  between  the  blocks  filled 
with  an  impermeable  cement. 

Experience  has  proved  beyond  doubt  that  this  latter  type  of 
pavement  is  the  most  enduring  and  economical  for  roadways  sub- 
jected to  heavy  and  constant  traffic.  Its  advantages  are  many, 
while  its  defects  are  few. 

Advantages. 

(1)  Adapted  to  all  grades. 

(2)  Suits  all  classes  of  traffic. 

(3)  Exceedingly  durable. 

(4)  Foothold,  fair. 

(5)  Requires  but  little  repair. 

(6)  Yields  but  little  dust  or  mud. 

(7)  Facility  for  cleansing,  fair. 
Defects. 

(1)  Under  certain  conditions  of  the  atmosphere,  the  surface  of 
the  pavement  becomes  greasy  and  slippery. 

(2)  The  incessant  din  and  clatter  occasioned  by  the  movement 
of  traffic  is  an  intolerable  nuisance;  it  is  claimed  by  many  physicians 
that  the  noise  injuriously  affects  the  nerves  and  health  of  persons 
who  are  obliged  to  live  or  do  business  in  the  vicinity  of  streets 
so  paved. 

(3)  Horses  constantly  employed  upon  it  soon  suffer  from  the 
continual  jarring  produced  in  their   legs  and   hoofs,  and  quickly 
wear  out. 


HIGHWAY  CONSTRUCTION  127 

(4)  The  discomfort  of  persons  riding  over  the  pavement  is  very 
great,  because  of  the  continual  jolting  to  which  they  are  subjected. 

(5)  If  stones  of  an  unsuitable  quality  are  used — for  example, 
those  that  polish — the  surface  quickly  becomes  slippery  and  exceed- 
ingly unsafe  for  travel. 

Blocks.  Size  and  Shape.  The  proper  size  of  blocks  for  paving 
purposes  has  been  a  subject  of  much  discussion,  and  a  great  variety 
of  forms  and  dimensions  are  to  be  found  in  all  cities. 

For  stability,  a  certain  proportion  must  exist  between  the  depth, 
the  length,  and  the  breadth.  The  depth  must  be  such  that  when  the 
wheel  of  a  loaded  vehicle  passes  over  one  edge  of  the  upper  surface 
of  a  block,  the  block  will  not  tend  to  tip  up.  The  resultant  direction 
of  the  pressure  of  the  load  and  adjoining  blocks  always  should  tend 
to  depress  the  whole  block  vertically ;  where  this  does  not  happen,  the 
maintenance  of  a  uniform  surface  is  impossible.  To  fulfill  this  re- 
quirement, it  is  not  necessary  to  make  the  block  more  than  6  inches 
deep. 

Width.  The  maximum  width  of  blocks  is  controlled  by  the 
size  of  horses'  hoofs.  To  afford  good  foothold  to  horses  drawing 
heavy  loads,  it  is  necessary  that  the  width  of  each  block,  measured 
along  the  street,  shall  be  the  least  possible  consistent  with  stability. 
If  the  width  be  great,  a  horse  drawing  a  heavy  load,  attempting  to  find 
a  joint,  slips  back,  and  requires  an  exceptionally  wide  joint  to  pull 
him  up.  It  is  therefore  desirable  that  the  width  of  a  block  shall  not 
exceed  3  inches;  or  that  four  blocks,  taken  at  random  and  placed 
side  by  side,  shall  not  measure  more  than  14  inches. 

Length.  The  length,  measured  across  the  street,  must  be 
sufficient  to  break  joints  properly,  for  two  or  more  joints  in  line  lead 
to  the  formation  of  grooves.  For  this  purpose  the  length  of  the 
block  should  be  not  less  than  9  inches  nor  more  than  12  inches. 

Form.  The  blocks  should  be  well  squared,  and  must  not  taper 
in  any  direction;  sides  and  ends  should  be  free  from  irregular  pro- 
jections. Blocks  that  taper  from  the  surface  downwards  (wedge- 
shaped)  should  not  be  permitted  in  the  work;  but  if  any  are  allowed, 
they  should  be  set  with  the  widest  side  down. 

Manner  of  Laying  Blocks.  The  blocks  should  be  laid  in  parallel 
courses,  with  their  longest  side  at  right  angles  to  the  axis  of  the 
street,  and  the  longitudinal  joints  broken  by  a  lap  of  at  least  2  inches, 


128 


HIGHWAY  CONSTRUCTION 


Quite 'r  Formed  of  3  Rows  of 
Blocks,  Set  Longitudinally 


Figs.  79  and  80.  The  reason  for  this  is  to  prevent  the  formation 
of  longitudinal  ruts,  which  would  happen  if  the  blocks  were  laid 
lengthwise.  Laying  blocks  obliquely  and  "herringbone"  fashion  has 

been  tried  in  several 
cities,  with  the  idea  that 
the  wear  and  formation 
of  ruts  would  be  reduced 
by  having  the  vehicle 
cross  the  blocks  diago- 
nally. The  method  has 
failed  to  give  satisfactory 


Cross  Section 

Fig.  79.     Section  Showing  Method  of  Laying  Ston< 
Block  Pavement 

results;    the    wear    was 

irregular  and  the  foothold  defective;  the  difficulty  of  construction 
was  increased  by  reason  of  labor  required  to  form  the  triangular 
joints;  and  the  method  was  wasteful  of  material. 

The  gutters  should 
be  formed  by  three  or 
more  courses  of  block, 
laid  with  their  length 
parallel  to  the  curb. 

At  junctions  or  inter- 
sections of  streets,  the 
blocks  should  be  laid 


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Plan 


Fig.  80. 


Plan  of  Stone-Block  Pavement  Showing  Method 
of  Laying  Blocks 


diagonally  from  the  cen- 
ter, as  shown  in  Fig.  81.  The  reasons  for  this  are:  (1)  to  prevent 
the  traffic  crossing  the  intersection  from  following  the  longitudinal 
joints  and  thus  forming  depressions  and  ruts;  (2)  laid  in  this  manner, 
the  blocks  afford  a  more  secure  foothold  for  horses  turning  the 
corners.  The  ends  of  the  diagonal  blocks  where  they  abut  against 
the  straight  blocks,  must  be  cut  to  the  required  bevel. 

The  blocks  forming  each  course  must  be  of  the  same  depth,  and 
no  deviation  greater  than  J  inch  should  be  permitted.  The  blocks 
should  be  assorted  as  they  are  delivered,  and  only  those  correspond- 
ing in  depth  and  width  should  be  used  in  the  same  course.  The 
better  method  would  be  to  gage  the  blocks  at  the  quarry.  This 
would  lessen  the  cost  considerably;  it  would  avoid  also  the  incon- 
venience to  the  public  due  to  the  stopping  of  travel  because  of  the 
rejection  of  defective  material  on  the  ground.  This  method  undoubt- 


HIGHWAY  CONSTRUCTION 


'129 


edly  would  be  preferable  to  the  contractor,  who  would  be  saved  the 
expense  of  handling  unsatisfactory  material;  and  it  also  would  leave 
the  inspectors  free  to  pay  more  attention  to  the  manner  in  which  the 
work  of  paving  is  performed. 

The  accurate  gaging  of  the  blocks  is  a  matter  of  much  impor- 
tance. If  good  work  is  to  be  executed,  the  blocks,  when  laid,  must  be 
in  parallel  and  even  courses;  and  if  the  blocks  are  not  gaged  accurately 
to  one  uniform  size,  the  result  will  be  a  badly  paved  street,  with  the 
courses  running  unevenly.  The  cost  of  assorting  blocks  into  lots  of 
uniform  width,  after  delivery  on  the  street,  is  far  in  excess  of  any 


Fig.  81.     Diagram  Showing  Method  of  Laying  Stone  Blocks  at 
Intersection  of  Streets 

additional  price  which  would  have  to  be  paid  for  accurate  gaging  at 
the  quarry. 

Foundation.  The  foundation  of  the  blocks  must  be  solid  and 
unyielding.  A  bed  of  hydraulic-cement  concrete  is  the  most  suitable, 
and  its  thickness  must  be  regulated  according  to  the  traffic;  the 
thickness,  however,  should  not  be  less  than  4  inches,  and  need  not  be 
more  than  9  inches.  A  thickness  of  6  inches  will  sustain  traffic  of 
600  tons  per  foot  of  width. 

Cushion  Coat.  Between  the  surface  of  the  concrete  and  the  base 
of  the  blocks,  there  must  be  placed  a  cushion  coat  formed  of  an 
incompressible  but  mobile  material,  the  particles  of  which  readily 
will  adjust  themselves  to  the  irregularities  of  the  bases  of  the  blocks 


130  HIGHWAY  CONSTRUCTION 

and  transfer  the  pressure  of  the  traffic  uniformly  to  the  concrete 
below.  A  layer  of  dry,  clean  sand  1  inch  to  2  inches  thick  forms  an 
excellent  cushion  coat.  Its  particles  must  be  of  such  fineness  as  to 
pass  through  a  No.  8  screen;  if  the  sand  is  coarse  and  contains  peb- 
bles, it  will  not  adapt  itself  to  the  irregularities  of  the  bases  of  the 
blocks;  hence  the  blocks  will  be  supported  at  a  few  points  only,  and 
unequal  settlement  will  take  place  when  the  pavement  is  subjected 
to  the  action  of  traffic.  The  sand  also  must  be  perfectly  free  from 
'moisture,  and  artificial  heat  must  be  used  to  dry  it  if  necessary. 
This  requirement  is  an  absolute  necessity.  There  should  be  no 
moisture  below  the  blocks  when  laid;  nor  should  water  be  allowed 
to  penetrate  below  the  blocks;  if  such  happens,  the  effect  of  frost 
will  be  to  upheave  the  pavement  and  crack  the  concrete. 

Where  the  best  is  desired  without  regard  to  cost,  a  layer  of 
asphaltic  cement  \  inch  thick  may  be  substituted  for  the  sand,  with 
superior  and  very  satisfactory  results. 

Laying  Blocks.  The  blocks  should  be  laid  stone  to  stone,  so  that 
the  joint  may  be  of  the  least  possible  width;  wide  joints  cause 
increased  wear  and  noise,  and  do  not  increase  the  foothold.  The 
courses  should  be  commenced  on  each  side  and  should  be  worked 
toward  the  middle;  and  the  last  stone  should  fit  tightly. 

Ramming.  After  the  blocks  have  been  set,  they  should  be  well 
rammed  down;  and  the  stones  which  sink  below  the  general  level 
should  be  taken  up  and  replaced  with  a  deeper  stone  or  brought  to 
level  by  increasing  the  sand  bedding. 

The  practice  of  workmen  invariably  is  to  use  the  rammer  so  as 
to  secure  a  fair  surface.  This  does  not  give  the  result  intended  to  be 
secured,  but  brings  each  block  to  an  unyielding  bearing.  The  result 
of  such  a  surfacing  process  is  to  produce  an  unsightly  and  uneven 
roadway  when  the  pressure  of  traffic  is  brought  upon  it.  The 
rammer  used  should  weigh  not  less  than  50  pounds  and  have  a 
diameter  of  not  less  than  3  inches. 

Fillings  for  Joints.  All  stone-block  pavements  depend  for  their 
waterproof  qualities  upon  the  character  of  the  joint  filling.  Joints 
filled  with  sand  and  gravel  of  course  are  pervious.  A  grout  of  lime 
or  cement  mortar  does  not  make  a  permanently  waterproof  joint;  it 
becomes  disintegrated  under  the  vibration  of  traffic.  An  impervious 
joint  can  be  made  only  by  employing  a  filling  made  from  bituminous 


HIGHWAY  CONSTRUCTION  131 

or  asphaltic  material;  this  renders  the  pavement  more  impervious 
to  moisture,  makes  it  less  noisy,  and  adds  considerably  to  its 
strength. 

Bituminous  Cement  for  Joint  Filling.  The  bituminous  materials 
employed  are:  (1)  coal  tar  having  a  specific  gravity  between  1.23 
and  1.33  at  60  degrees  Fahrenheit,  a  melting  point  between  120  and 
130  degrees  Fahrenheit,  and  containing  not  over  30  per  cent  of  free 
carbon.  (2)  asphalt,  either  natural  or  artificial,  entirely  free  from 
coal  tar  or  any  product  of  coal-tar  distillation,  and  containing  not 
less  than  98  per  cent  of  pure  bitumen  soluble  in  carbon  bisulphide. 
Of  the  total  amount  soluble  in  carbon  bisulphide,  98.5  per  cent  must 
be  soluble  in  carbon  tetrachloride.  The  penetration,  when  tested 
by  the  Dow  method,  must  be  not  greater  than  110,  at  115  degrees 
Fahrenheit,  and  at  77  degrees  Fahrenheit  must  range  between  25 
and  60.  The  specific  gravity  at  60  degrees  Fahrenheit  must  not  be 
more  than  1.00. 

The  mode  of  applying  the  coal-tar  filler  is  as  follows :  After  the 
blocks  are  laid,  gravel  heated  to  about  250  degrees  Fahrenheit 
is  spread  over  the  surface  and  swept  into  the  joints  until  they  are 
filled  to  a  depth  of  about  2  inches.  The  blocks  then  are  rammed. 
The  coal-tar  filler  heated  to  a  temperature  between  250  and  300 
degrees  Fahrenheit  is  poured  into  the  joints  until  they  are  about 
half  filled,  hot  gravel  is  swept  in  until  it  reaches  to  within  |  inch 
of  the  surface,  and  hot  filler  is  then  poured  in  until  it  is  flush  with 
the  surface  of  the  blocks;  after  this  sufficient  hot  gravel  is  applied 
to  the  joints  to  conceal  the  filler. 

In  applying  the  coal-tar  filler  it  is  essential  that  both  the  gravel 
and  filler  are  heated  sufficiently.  Otherwise  the  filler  will  be  chilled 
and  will  not  flow  to  the  bottom  of  the  joint,  but  will  form  a  thin  layer 
near  the  surface,  which  under  the  action  of  frost  and  the  vibration 
of  traffic,  will  be  cracked  and  broken  up  quickly;  the  gravel  will 
settle,  and  the  blocks  will  be  jarred  loose,  causing  the  surface  of 
the  pavement  to  become  a  series  of  ridges  and  hollows.  The  filler 
should  not  be  applied  during  a  rainfall  or  while  the  blocks  are  wet 
or  damp,  for  such  a  condition  would  prevent  the  filler  from  adhering 
to  the  blocks.  The  asphalt  filler  is  heated  to  a  temperature  between 
400  and  450  degrees  Fahrenheit  and  poured  into  the  joints  until 
they  are  entirely  filled. 


132 


HIGHWAY  CONSTRUCTION 


Hydraulic-Cement  Filler  is  composed  of  equal  parts  of  Portland 
cement  and  sharp  sand  mixed  with  clean  fresh  water  to  a  suitable 
consistency.  The  joints  between  the  blocks  are  filled  to  a  depth 
of  2  inches  with  gravel,  and  the  blocks  are  rammed,  after  which 
the  filler  is  poured  into  the  joints  until  they  are  filled  flush  with  the 
surface  of  the  blocks.  In  dry  weather  the  blocks  should  be  mois- 
tened by  sprinkling  with  water  before  applying  the  filler.  After 
the  filler  has  taken  its  initial  set,  the  whole  surface  of  the  pavement 
is  covered  with  a  layer  of  sand  about  J  inch  thick  and  if  the  weather 
is  dry  and  warm  it  is  sprinkled  with  water  daily  for  three  days. 
Traffic  is  not  permitted  to  use  the  pavement  until  at  least  seven 
days  after  completion. 

Stone  Pavement  on  Steep  Grades.  Stone  blocks  may  be 
employed  on  all  practicable  grades,  but  on  grades  exceeding  10 

per  cent,  cobblestones 
afford  a  better  foothold 
than  blocks.  The  cob- 
blestones should  be  of  uni- 
form length,  the  length 
being  at  least  twice  the 
breadth — say  stones  6 
inches  long  and  2|  inches 
to  3  inches  in  diameter. 
These  should  be  set  on  a  concrete  foundation,  laid  stone  to  stone, 
and  the  interstices  filled  with  cement  grout  or  bituminous  cement; 
or  a  bituminous-concrete  foundation  may  be  employed  and  the 

interstices  between  the 
stones  may  be  filled  with 
asphaltic  paving  cement. 
Should  stone  blocks  be 
preferred,  they  must  be 
laid,  when  the  grade  ex- 
ceeds 5  per  cent,  with  a 
serrated  surface,  by  either 
of  the  methods  shown  in 
Figs.  82  and  83.  The  method  shown  in  Fig.  82  consists  in  slightly 
tilting  the  blocks  on  their  bed  so  as  to  form  a  series  of  ledges  or 
steps,  which  will  insure  a  good  foothold  for  horses'  hoofs.  The 


Fig.  82. 


Laying  Stone  Pavement  on  Steep  Grades  by 
Tilting  Blocks 


Fig.  83.     Laying   Stone   Pavement  on   Steep  Grades  by 
Separating  Blocks  and  Filling  with  Grout 


HIGHWAY  CONSTRUCTION 


133 


method  shown  in  Fig.  83  consists  in  placing  between  the  rows  of 
stones  a  course  of  slate,  or  strips  of  creosoted  wood,  rather  less 
than  1  inch  in  thickness  and  about  1  inch  less  in  depth  than  the 
blocks;  or  the  blocks  may  be  spaced  about  1  inch  apart,  and  the 
joints  filled  with  a  grout  composed  of  gravel  and  cement.  The 
pebbles  of  the  gravel  should  vary  in  size  between  f  inch  and  f  inch, 

BRICK  PAVEMENTS 

A  brick  pavement  consists  of  vitrified  bricks  laid  on  a  suitable 
concrete  foundation,  Fig.  84. 

Qualifications  of  Brick.  The  qualities  essential  to  a  .good 
paving  brick  are  the  same  as  for  any  other  paving  material,  viz, 
hardness,  toughness,  and  ability  to  resist  the  disintegrating  effects 


Vitrified  Brick- 


Fig.  84.     Section  Showing  Method  of  Laying  Vitrified  Brick  Pavement 

of  water  and  frost.  These  qualities  are  imparted  to  the  brick  by 
a  process  of  annealing,  through  which  the  clay  is  brought  to  the 
point  of  fusion,  and  the  heat  then  gradually  reduced  until  the  kiln 
is  cold. 

Composition.  The  material  from  which  is  made  the  majority 
of  the  brick  used  for  paving  is  a  shale.  Shales  are  indurated  clays 
with  a  laminated  structure  and  the  appearance  of  slate,  and  occur 
in  stratified  beds.  The  average  composition  of  the  shales  that 
have  proved  satisfactory  for  the  manufacture  of  paving  brick  is 
shown  in  Table  XIV. 

An  excess  of  silica  causes  brittleness;  or  an  excess  of  alumina 
causes  shrinking,  cracking,  and  warping.  Iron  renders  the  clay 
fusible  and  makes  the  brick  more  homogeneous.  Lime  in  the  form 
of  silicate  is  valuable  as  a  flux,  but  in  the  form  of  carbonate  it  will 


134 


HIGHWAY  CONSTRUCTION 


TABLE  XIV 

Average  Composition  of  Shales  for  Paving- 
Brick  Manufacture 


CONSTITUENTS 

PROPORTIONAL 
PART 
(per  cent) 

(Non-Fluxing) 

Silica      . 
Alumina 
Water  and  loss  on  ignition 
Moisture 

56.0 
22.0 
7.0 
2.0 

(Fluxing) 
Sesquioxide  of  iron 
Lime 
Magnesia 
Alkalies 

7.0 
1.0 
1.0 
4.0 

Total 

100.0 

decrease  the  strength  of  the  brick;  at  a  high  temperature  it  is  changed 
into  caustic  lime,  which,  while  rendering  the  clay  more  fusible,  will 
absorb  moisture  upon  exposure  to  the  weather  and  thus  cause  the 
brick  to  disintegrate.  Magnesia  exerts  but  little  influence  on  the 
character  of  the  brick.  The  alkalies  in  small  quantities  render 
the  clay  fusible. 

Color.  The  color  of  the  clay  is  of  no  practical  importance; 
it  is  due  to  the  presence  of  the  metallic  oxides  and  organic  substances. 
Iron  produces  bricks  which  are  either  red,  yellow,  or  blue,  according 
to  the  quantity  present  and  the  degree  of  heat;  some  organic  sub- 
stances produce  a  blue,  bluish-gray,  or  black  color. 

The  color  of  the  brick  is  governed  partly  by  the  color  of  the 
clay,  by  the  temperature  of  burning,  by  the  kind  of  fuel  used,  and 
by  the  sand  that  is  used  to  prevent  the  brick  from  sticking  to  the 
dies  or  to  each  other  in  the  kiln. 

Manufacture.  In  the  manufacture  of  the  brick,  the  shale  is 
crushed  usually  in  dry  paws  and  then  passed  through  a  4-mesh  or 
an  8-mesh  screen.  The  screened  material  is  mixed  with  water  in 
a  pug  mill  to  the  required  consistency.  The  finer  the  material 
is  crushed  and  the  more  thoroughly  it  is  worked  or  tempered  in 
the  mill,  the  more  uniform  and  better  the  brick  is. 

The  plastic  clay,  in  the  "stiff-mud"  process,  as  it  leaves  the  pug 
mill  is  forced  by  an  auger  through  a  die  which  forms  a  bar  of  stiff 


HIGHWAY  CONSTRUCTION  135 

clay  of  the  desired  dimensions,  and  this  is  cut  by  an  automatic  cutter 
into  bricks  of  the  size  required.  The  bricks  then,  in  some  factories, 
are  repressed  in  a  die,  during  which  the  edges  of  the  brick  are  rounded 
and  the  lugs,  grooves,  and  trade-mark  stamped  on  the  sides.  When 
repressing  is  not  practiced,  the  bar  of  clay  as  it  comes  from  the  pug 
mill  is  cut  by  wires,  the  brick  being  called  "wire-cut  lug"  brick. 

The  bricks,  made  by  either  method,  are  placed  in  a  heated 
chamber  to  dry,  this  requiring  from  18  to  60  hours  according  to  the 
clay,  temperature,  and  plant  arrangement.  When  dry  the  bricks 
are  stacked  in  the  kiln,  which  is  usually  of  the  down-draft  type  with 
furnaces  built  in  the  outer  wralls.  The  bottom  of  the  kiln  is  perfo- 
rated to  allow  the  gases  to  pass  through  to  the  flues  placed  below  the 
floor  and  connected  to  the  chimney.  The  heat  from  the  furnace 
passes  upward  into  the  kiln,  then  downward  through  the  bricks 
into  the  flues  and  thence  to  the  chimney.  At  the  beginning  of  the 
burning  the  heat  is  applied  slowly  to  drive  off  the  contained  water 
without  cracking  the  bricks.  When  the  dryness  of  the  smoke 
indicates  the  absence  of  moisture  in  the  bricks,  the  fires  are  gradually 
increased  until  the  temperature  throughout  the  kiln  is  from  1500 
to  2000  degrees  Fahrenheit,  this  temperature  being  maintained  from 
seven  to  ten  days.  The  kiln  then  is  closed,  the  fires  are  drawn,  and 
the  bricks  are  allowed  to  cool.  This  part  of  the  process  is  called 
annealing,  and  to  produce  a  tough  brick  requires  from  seven  to  ten 
days.  The  cooled  bricks  are  sorted  into  different  lots;  the  No.  1 
paving  bricks  are  generally  found  in  the  upper  layers  in  the  kiln. 

Sizes.  Two  sizes  of  bricks  are  made:  one  size  measuring  8JX 
2|  X  4  inches  weighing  about  7  pounds  and  requiring  58  to  the  square 
yard.  The  other,  measuring  8  J  X  3  J  X  4  inches  and  frequently  called 
"blocks",  weighs  about  9J  pounds  and  requires  45  to  the  square 
yard. 

Characteristics.  The  characteristics  of  brick  suitable  for  paving 
are :  not  to  be  acted  upon  by  acids — shale  bricks  not  to  absorb  more 
than  2  per  cent  nor  less  than  ^  of  1  per  cent  of  their  weight  of  water, 
and  clay  bricks  not  to  absorb  more  than  6  per  cent  of  their  weight 
of  water  (the  absorption  by  a  shale  brick  of  less  than  |  of  1  per  cent 
of  its  weight  of  water,  indicates  that  it  has  been  overburned) ;  when 
broken  with  a  hammer,  to  show  a  dense  close-grained  structure?  free 
from  lime,  air  holes,  cracks,  or  marked  laminations;  not  to  scale, 


136  HIGHWAY  CONSTRUCTION 

spall,  or  chip,  when  quickly  struck  on  the  edges;  hard  but  not 
brittle. 

Tests  for  Paving  Brick.  To  ascertain  if  brick  possesses  the 
required  qualities  they  are  subjected  to  three  tests:  (1)  abrasion 
by  impact  (commonly  called  the  "rattler"  test);  (2)  absorption; 
(3)  cross  breaking. 

The  Rattler  Test.  The  rattler  is  a  steel  barrel  28  inches  long  and 
28  inches  in  diameter,  the  sides  formed  of  14  staves  fastened  to  two 
cast-iron  heads  furnished  with  trunnions  which  rest  in  a  cast-iron 
frame.  It  is  provided  with  gears  and  a  belt  pulley  arranged  to 
revolve  at  a  rate  of  from  29  J  to  30 J  revolutions  per  minute.  The 
material  employed  to  abrade  the  brick  is  spherical  balls  of  cast  iron, 
the  composition  of  which  is:  combined  carbon,  not  less  than  2.50  per 
cent;  graphitic  carbon,  not  more  than  0.10  per  cent;  silicon,  not  more 
than  1  per  cent;  manganese,  not  more  than  0.50  per  cent;  phos- 
phorus, not  more  than  0.25  per  cent;  sulphur,  not  more  than  0.08 
per  cent.  Two  sizes  of  balls  or  shot  are  used,  the  larger  being  3.75 
inches  in  diameter  when  new  and  weighing  about  7J  pounds,  the 
smaller  being  1.875  inches  in  diameter  and  weighing  0.95  pounds. 
A  charge  consists  of  ten  large  shot  with  enough  small  shot  to  make 
a  weight  of  300  pounds.  The  shot  is  used  until  the  large  size  is  worn 
to  a  weight  of  7  pounds  and  the  small  shot  is  worn  to  a  size  that  will 
pass  through  a  circular  hole  If  inches  in  diameter  made  in  a  cast- 
iron  plate  J-inch  thick. 

The  brick  to  be  tested  are  subjected  to  a  temperature  of  100 
degrees  Fahrenheit  for  three  hours.  Ten  bricks  are  weighed  and 
placed  in  the  rattler  with  a  charge  of  spherical  shot,  and  the  rattler 
is  revolved  for  1800  revolutions.  The  bricks  then  are  taken  out, 
pieces  less  than  1  pound  in  weight  are  removed  and  the  balance 
weighed.  From  the  weights  before  and  after  rattling  the  percent- 
age of  loss  is  calculated.  The  loss  ranges  from  16  per  cent  to  40  per 
cent.  Brick  to  be  used  under  heavy  traffic  should  not  lose  more  than 
22  per  cent,  and  for  light  traffic  not  more  than  28  per  cent. 

Absorption  Test.  The  absorption  test  is  made  on  five  bricks 
that  have  been  through  the  rattler  test.  They  are  weighed,  and 
are  immersed  in  water  for  48  hours,  then  are  taken  out  and  weighed, 
with  the  surplus  water  wiped  off.  From  the  weights  before  and  after 
immersion  the  percentage  of  water  absorbed  is  calculated. 


HIGHWAY  CONSTRUCTION  137 

Cross-Breaking  Test.  This  test  is  made  by  placing  a  brick  edge 
on  supports  6  inches  apart.  The  load  is  applied  at  the  center  of 
the  brick,  and  is  increased  uniformly  until  fracture  occurs.  The 
average  of  the  result  on  ten  bricks  is  used  in  computing  the  modulus 

OTJ77 

of  rupture,  R=—r^-',  in  which  IF  is  the  average  breaking  load  in 


pounds,  L  the  length  between  supports  in  inches,  b  the  breadth, 
and  d  the  depth  in  inches. 

Brick=Pavement  Qualifications.  Advantages.  The  advantages 
of  brick  pavement  may  be  stated  as  follows: 

(1)  Easy  traction. 

(2)  Good  foothold  for  horses. 

(3)  Not  disagreeably  noisy. 

(4)  Yields  but  little  dust  and  mud. 

(5)  Adapted  to  all  grades. 

(6)  Easily  repaired. 

(7)  Easily  cleaned. 

(8)  But  slightly  absorbent. 

(9)  Pleasing  to  the  eye. 

(10)  Expeditiously  laid. 

(11)  Durable  under  moderate  traffic. 

Defects.  The  principal  defects  of  brick  pavements  arise  from 
lack  of  uniformity  in  the  quality  of  the  bricks,  and  from  the  liability 
of  incorporating  in  the  pavement  bricks  too  soft  or  too  porous  a 
structure,  which  crumbles  under  the  action  of  traffic  or  frost. 

Foundation.  A  brick  pavement  should  have  a  firm  foundation. 
As  the  surface  is  made  up  of  small,  independent  blocks,  each  one 
must  be  supported  adequately,  or  the  load  coming  upon  it  may  force 
it  downwards  and  cause  unevenness,  a  condition  which  conduces 
to  the  rapid  destruction  of  the  pavement.  Several  forms  of  founda- 
tion have  been  used  —  such  as  gravel,  plank,  sand,  broken  stone, 
and  concrete.  The  last  mentioned  is  the  best. 

Sand  Cushion.  The  sand  cushion  is  a  layer  of  sand  placed 
on  top  of  the  concrete  to  form  a  bed  for  the  brick.  Practice  regard- 
ing the  depth  of  this  layer  of  sand  varies  considerably.  In  some 
cases  it  is  only  |  inch  deep,  varying  from  this  up  to  3  inches.  The 
sand  cushion  is  very  desirable,  as  it  not  only  forms  a  perfectly 
true  and  even  surface  upon  which  to  place  brick,  but  also  makes  the 


138  HIGHWAY  CONSTRUCTION 

pavement  less  hard  and  rigid  than  would  be  the  case  were  the  brick 
laid  directly  on  the  concrete. 

The  sand  is  spread  evenly,  sprinkled  with  water,  smoothed, 
and  brought  to  the  proper  contour  by  screeds  or  wooden  templets, 
properly  trussed  and  mounted  on  wheels  or  shoes  which  bear  upon 
the  upper  surface  of  the  curb.  Moving  the  templet  forward  levels 
and  forms  the  sand  to  a  uniform  surface  and  proper  shape. 

The  sand  used  for  the  cushion  coat  should  be  clean  and  free  from 
loam,  moderately  coarse,  and  free  from  pebbles  exceeding  J  inch 
in  size. 

Manner  of  Laying.  The  bricks  should  be  laid  on  edge  or  on 
one  flat,  as  closely  and  compactly  as  possible,  in  straight  courses 
across  the  street,  with  the  length  of  the  bricks  at  right  angles  to 
the  axis  of  the  street.  Joints  should  be  broken  by  at  least  3  inches. 
None  but  whole  bricks  should  be  used,  except  in  starting  a  course 
or  making  a  closure.  To  provide  for  the  expansion  of  the  pavement, 
both  longitudinal  and  transverse  expansion  joints  are  used,  the 
former  being  made  by  placing  a  board  templet  J-inch  thick 
against  the  curb  and  abutting  the  brick  thereto.  The  transverse 
joints  are  formed  at  intervals  varying  between  25  and  50  feet,  by 
placing  a  templet  or  building  lath  f-inch  thick  between  two  or  three 
rows  of  brick.  After  the  bricks  are  rammed  and  ready  for  grouting, 
these  templets  are  removed,  and  the  spaces  so  left  are  filled  with 
coal-tar  pitch  or  asphaltic  paving  cement.  The  amount  of  pitch 
or  cement  required  will  vary  between  1  and  1J  pounds  per  square 
yard  of  pavement,  depending  upon  the  width  of  the  joints.  After 
25  or  30  feet  of  the  pavement  is  laid,  every  part  of  it  should  be  rammed 
with  a  rammer  weighing  not  less  than  50  pounds  and  the  bricks 
which  sink  below  the  general  level  should  be  removed,  sufficient 
sand  being  added  to  raise  the  brick  to  the  required  level.  After 
all  objectionable  brick  have  been  removed,  the  surface  should  be 
swept  clean,  then  rolled  with  a  steam  roller  weighing  from  3  to  6 
tons.  The  object  of  rolling  is  to  bring  the  bricks  to  an  unyielding 
bearing  with  a  plane  surface;  if  this  is  not  done,  the  pavement 
will  be  rough  and  noisy  and  will  lack  durability.  The  rolling  should 
be  executed  first  longitudinally,  beginning  at  the  crown  and  working 
toward  the  gutter,  taking  care  that  each  return  trip  of  the  roller 
covers  exactly  the  same  area  as  the  preceding  trip,  so  that  the  second 


HIGHWAY  CONSTRUCTION 


139 


passage  may  neutralize  any  careening  of  the  brick. due  to  the  first 
passage. 

The  manner  of  laying  brick  at  street  intersections  is  shown  in 
Fig.  85. 

Joint  Fillings.  The  character  of  the  material  used  in  filling 
the  joints  between  the  brick  has  considerable  influence  on  the  success 


Fig.  85.     Method  of  Laying  Bricks  at  Street  Intersections 

and  durability  of  the  pavement.  Various  materials  have  been 
used — such  as  sand,  coal-tar  pitch,  asphalt,  mixtures  of  coal  tar 
and  asphalt,  and  Portland  cement,  besides  various  patented  fillers, 
as  "Murphy's  grout",  which  is  made  from  ground  slag  and  cement. 
Each  material  has  its  advocates,  and  there  is  much  difference  of 
opinion  as  to  which  gives  the  best  results. 

The  best  results  seem  to  be  obtained  by  using  a  high  grade  of 
Portland  cement  containing  the  smallest  amount  of  lime  in  its 


140  HIGHWAY  CONSTRUCTION 

composition;  the  presence  of  the  lime  increasing  the  tendency  of 
the  filler  to  swell  through  absorption  of  moisture,  causing  the  pave- 
ment to  rise  or  to  be  lifted  away  from  its  foundation,  and  thus 
producing  the  roaring  or  rumbling  noise  so  frequently  complained  of. 
The  Portland-cement  grout,  when  uniformly  mixed  and  care- 
fully placed,  resists  the  impact  of  traffic  and  wears  well  with  brick. 
When  a  failure  occurs,  repairs  can  be  made  quickly,  and,  if  made 
early,  the  pavement  will  be  restored  to  a  good  condition.  If,  how- 
ever, repairs  are  neglected,  the  brick  soon  loosen  and  the  pave- 
ment fails. 


Fig.  86.     Grout  Box  Used  in  Laying  Brick  Pavement 
Courtesy  of  National  Paving  Brick  Manufacturers  Association,  Cleveland,  Ohio 

The  office  of  a  filler  is  to  prevent  water  from  reaching  the  founda- 
tion, and  to  protect  the  edges  of  the  brick  from  spalling  under 
traffic.  In  order  to  meet  both  of  these  requirements,  every  joint 
must  be  filled  to  the  top,  and  must  remain  so,  wearing  down  with 
the  brick.  Sand  does  not  meet  these  requirements.  Although  at 
first  making  a  good  filler,  being  inexpensive  and  reducing  the  liability 
of  the  pavement  to  be  noisy,  it  soon  washes  out,  leaving  the  edges 
of  the  brick  unprotected  and  consequently  liable  to  be  chipped. 
Coal  tar  and  the  mixtures  of  coal  tar  and  asphalt  have  an  advantage 
in  rendering  a  pavement  less  noisy  and  in  cementing  together  any 
breaks  that  may  occur  through  upheavals  from  frost  or  other  causes; 


HIGHWAY  CONSTRUCTION  141 

but,  unless  made  very  hard,  they  have  the  disadvantage  of  becoming 
soft  in  hot  weather  and  flowing  to  the  gutters  and  low  places  in 
the  pavement,  there  forming  a  black  and  unsightly  scale  and  leaving 
the  high  parts  unprotected.  The  joints,  thus  deprived  of  their  filling, 
become  receptacles  for  water,  mud,  and  ice  in  turn;  and  the  edges 
of  the  brick  are  broken  down  quickly.  Some  of  these  mixtures 
become  so  brittle  in  winter  that  they  crack  and  fly  out  of  the  joints 
under  the  action  of  traffic. 

The  Portland-cement  filler  is  prepared  by  mixing  2  parts  of 
cement  and  1  part  of  fine  sand  with  sufficient  water  to  make  a  thin 
grout.  The  most  convenient  arrangement  for  preparing  and  dis- 
tributing the  grout  is  a  water-tight  wooden  box  carried  on  four 
wood  wheels  about  12  inches  in  diameter,  Fig.  86.  The  box 
may  be  about  4  feet  wide,  7  feet  long,  and  12  inches  deep,  furnished 
with  a  gate  about  8  inches  wide,  in  the  rear  end.  The  box  should 
be  mounted  on  the  wheels  with  an  inclination,  so  that  the  rear  end 
is  about  4  inches  lower  than  the  front  end. 

Following  are  the  successive  operations  of  placing  the  filler: 
The  cement  and  sand  are  placed  in  the  box,  and  sufficient  water  is 
added  to  make  a  thin  grout.  The  grouting  box  is  located  about 
12  feet  from  the  gutter,  the  end  gate  opened,  and  about  2  cubic 
feet  of  the  grout  allowed  to  flow  out  and  run  over  the  top  of  the 
brick  (care  being  taken  to  stir  the  grout  while  it  is  being  dis- 
charged), Fig.  87.  If  the  brick  are  very  dry,  the  entire  surface  of 
the  pavement  should  be  wet  thoroughly  with  a  hose  before  applying 
the  grout;  if  not,  absorption  of  the  water  from  the  grout  by  the 
bricks  will  prevent  adhesion  between  the  bricks  and  the  cement 
grout.  The  grout  is  swept  into  the  joints  by  ordinary  bass  brooms. 
After  a  length  of  about  100  feet  of  the  pavement  has  been  covered 
the  box  is  returned  to  the  starting  point,  and  the  operation  is 
repeated  with  a  grout  somewhat  thicker  than  the  first.  If  this  second 
application  is  not  sufficient  to  fill  the  joints,  the  operation  is  repeated 
as  often  as  may  be  necessary  to  fill  them.  If  the  grout  has  been 
made  too  thin,  or  the  grade  of  the  street  is  so  great  that  the  grout 
will  not  remain  long  enough  in  place  to  set,  dry  cement  may  be 
sprinkled  over  the  joints  and  swept  in.  After  the  joints  are  filled 
completely  and  inspected,  allowing  three  or  four  hours  to  intervene, 
the  completed  pavement  should  be  covered  with  sand  to  a  depth 


7\ 


HIGHWAY  CONSTRUCTION 


143 


of  about  J  inch,  and  the  roadway  barricaded,  and  no  traffic  allowed 
on  it  for  at  least  ten  days. 

The  object  of  covering  the  pavement  with  sand  is  to  prevent 
the  grout  from  drying  or  settling  too  rapidly;  hence,  in  dry  and 
windy  weather,  it  should  be  sprinkled  from  time  to  time.  If  coarse 
sand  is  employed  in  the  grout,  it  will  separate  from  the  cement 
during  the  operation  of  filling  the  joints,  with  the  result  that  many 
joints  will  be  filled  with  sand  and  very  little  cement,  while  others 
will  be  filled  with  cement  and  little  or  no  sand;  thus  there  will  be 


Fig.  88.     Coal-Tar  Heating  Tank 

Courtesy  of  Barber  Asphalt  Paving  Company, 

Philadelphia,  Pennsylvania 

many  spots  in  the  pavement  in  which  no  bond  is  formed  between 
the  bricks,  and  under  the  action  of  traffic  these  portions  quickly 
will  become  defective. 

The  coal-tar  filler  is  best  applied  by  pouring  the  material  from 
buckets,  and  brooming  it  into  the  joints  with  wire  brooms;  and  in 
order  to  fill  the  joints  effectually,  it  must  be  used  only  when  very 
hot.  To  secure  this  condition,  a  heating  tank  on  wheels  is  necessary, 
Fig.  88.  It  should  have  a  capacity  of  at  least  5  barrels,  and  be  kept 
at  a  uniform  temperature  all  day.  One  man  is  necessary  to  feed 
the  fire  and  draw  the  material  into  the  buckets;  another,  to  carry 


HIGHWAY  CONSTRUCTION  145 

the  buckets  from  the  heating  tank  to  a  third,  who  pours  the  material 
over  the  street.  The  latter  starts  to  pour  in  the  center  of  the  street, 
working  backward  toward  the  curb,  and  pouring  a  strip  about  2  feet 
in  width.  A  fourth  man,  with  a  wire  broom,  follows  immediately 
after  him,  sweeping  the  surplus  material  toward  the  pourer  and  in 
the  direction  of  the  curb.  This  method  leaves  the  entire  surface 
of  the  pavement  covered  with  a  thin  coating  of  pitch,  which 
immediately  should  be  covered  with  a  light  coating  of  sand,  the 
sand  becoming  imbedded  in  the  pitch.  Under  the  action  of  traffic, 
this  thin  coating  is  worn  away  quickly,  leaving  the  surface  of  the 
bricks  clean  and  smooth,  Fig.  89. 

Tools  Used  by  Hand  in  the  Construction  of  Block  Pavements. 
The  principal  tools  required  in  constructing  block  pavements  com- 
prise hammers  and  rammers  of  varying  sizes  and  shapes,  depending 
on  the  material  and  size  of  the  blocks  to  be  laid;  also  crcnvbars,  sand 
screens,  and  rattan  and  wire  brooms.  Cobblestones,  square  blocks, 
and  brick  require  different  types  both  of  hammer  and  rammers 
for  adjusting  them  to  place  and  for  forcing  them  to  their  seats. 
A  cobblestone  rammer,  for  example,  is  usually  made  of  wood  (gener- 
ally locust)  in  the  shape  of  a  long  truncated  cone,  banded  with 
iron  at  top  and  bottom,  weighing  about  40  pounds,  and  having 
two  handles,  one  at  the  top  and  another  on  one  side.  A  Belgian- 
block  rammer  is  slightly  heavier,  consisting  of  an  upper  part  of 
wood  set  in  a  steel  base;  while  a  rammer  for  granite  blocks  is  still 
heavier,  comprising  an  iron  base  with  cast-steel  face,  into  which 
is  set  a  locust  plug  with  hickory  handles.  For  laying  brick,  a 
wooden  rammer  shod  with  cast  iron  or  steel  and  weighing  about  27 
pounds  is  used.  A  light  rammer  of  about  20  pounds  weight,  consist- 
ing of  a  metallic  base  attached  to  a  long,  slim,  wooden  handle,  is 
used  for  miscellaneous  work,  such  as  tamping  in  trenches,  next  to 
curbs,  etc. 

Concrete=Mixing  Machine.  Where  large  quantities  of  concrete 
are  required,  as  in  the  foundations  of  improved  pavements,  concrete 
can  be  prepared  more  expeditiously  and  economically  by  the  use  of 
mechanical  mixers,  and  the  ingredients  will  be  mixed  more  thoroughly 
than  by  hand.  Thorough  incorporation  of  the  ingredients  is  an 
essential  element  in  the  quality  of  a  concrete.  When  mixed  by 
hand,  however,  the  incorporation  is  rarely  complete,  because  it 


146  HIGHWAY  CONSTRUCTION 

depends  upon  the  proper  manipulation  of  the  hoe  and  shovel.  The 
manipulation,  although  extremely  simple,  is  rarely  performed  by 
the  ordinary  laborer  as  it  should  be  unless  he  is  watched  constantly 
by  the  overseer. 

Several  varieties  of  concrete-mixing  machines  are  in  the  market, 
all  of  which  are  efficient  and  of  good  design.     A  convenient  portable 


Fig.  90.     Smith  Concrete  Mixer  on  Truck  with  Gasoline  Engine, 

Power  Charger,  and  Water  Tank 
Courtesy  of  T.  L.  Smith  Company,  Milwaukee,  Wisconsin 

type  is  illustrated  in  Fig.  90.  The  capacity  of  the  mixers  ranges 
from  5  to  20  cubic  yards  per  hour,  depending  upon  size,  regularity 
with  which  the  materials  are  supplied,  speed,  etc. 

Gravel  Heaters.  A  special  type  of  oven  usually  is  employed 
for  heating  the  gravel  used  for  joint  filling  in  stone-block  pavements. 
These  heaters  are  made  in  various  sizes,  a  common  size  being  9  feet 
long,  5  feet  wide,  and  3  feet  9  inches  high. 


HIGHWAY  CONSTRUCTION  147 

WOOD=BLOCK  PAVEMENTS 

Wood-block  pavements,  Fig.  91,  are  formed  of  rectangular  blocks 
measuring  from  3J  to  4  inches  wide,  5  to  10  inches  long,  and  4  inches 
deep,  impregnated  with  creosote,  or  other  preservative,  laid  in  a 
bed  of  Portland-cement  mortar  spread  upon  a  concrete  foundation, 
with  the  joints  between  the  blocks  filled  with  either  Portland-cement 
grout,  or  a  bituminous  filler. 

The  wood  used  is  obtained  from  the  long-leaved  yellow  pine 
(pinus  palustrus),  lob-lolly  pine  (pinus  taeda),  short-leaved  pine 
(pinus  echinata),  Cuba  pine  (pinus  heterophylla),  black  gum  (nyssa 
sylvatica),  red  gum  (liquidambar  styrraciflua),  Norway  pine  (pinus 
resinosa),  or  tamarack  (larix  laricina). 

The  wood  should  be  cut  from  sound  trees,  free  from  cracks, 
snakes,  and  knots. 

The  great  enemy  of 
wood  pavement  is  decay 
due  to  a  low  form  of 
plant  life  called  fungi. 
The  fungi  attack  the 
wood  from  the  outside, 
and  if  the  wood  is  in  the 

right    Condition    for    the       Fig  91>     Section  Showing  Foundation  for  Wood-Block 

spores  to  grow,  they  ulti- 
mately will  penetrate  the  entire  structure  of  the  wood.  There 
are  three  classes  of  fungi:  one  which  attacks  all  parts  of  the 
wood  structure;  another  which  attacks  the  cellulose;  and  a  third, 
which  is  the  most  common,  and  attacks  only  the  lignin — the  name 
of  the  many  organic  substances  that  are  incrusted  around  the  cellu- 
lose, and  which  with  the  latter  constitute  the  essential  part  of  woody 
tissue — here  the  fungi  dissolve  the  lignin  and  the  cellulose  to  make 
food  for  their  development.  Heat,  air,  and  moisture  are  necessary 
to  the  existence  of  the  fungi;  without  any  one  of  these  elements 
they  cannot  live.  To  destroy  the  fungus  life  and  preserve  wood 
from  decay  many  processes  have  been  devised;  the  one  that  seems 
to  meet  the  requirements  better  than  any  other  is  the  process  of 
creosoting. 

Creosoting.  This  process  consists  in  impregnating  the  wood 
with  the  dead  oil  of  tar,  called  "creosote",  from  which  the  ammonia 


148  HIGHWAY  CONSTRUCTION 

has  been  removed.  Its  effect  on  the  wood  is  to  coagulate  the  albu- 
men and  thereby  prevent  its  decomposition,  also  to  fill  the  pores  of 
the  wood  with  a  bituminous  substance  which  excludes  both  air  and 
moisture,  and  which  is  obnoxious  to  the  lower  forms  of  animal  and 
vegetable  life. 

The  coal-tar  creosote  oil  is  used  without  admixture  or  adultera- 
tion with  other  oils  or  tars.  Its  characteristics  are :  specific  gravity, 
1.03  to  1.08,  at  a  temperature  of  100  degrees  Fahrenheit;  contain 
not  more  than  5  per  cent  of  tarry  matter,  nor  more  than  2  per  cent 
of  water,  and  not  more  than  8  per  cent  of  tar  acids,  99  per  cent  to 
be  soluble  in  hot  benzol ;  when  subjected  to  distillation  at  gradually 
increasing  temperatures  up  to  400  degrees  Fahrenheit  not  more  than 

5  per  cent  of  distillate  shall  pass  over,  at  450  degrees  not  more  than 
35  per  cent,  and  up  to  600  degrees  Fahrenheit  not  more  than  80  per 
cent;  after  complete  distillation  not  more  than  2  per  cent  of  coke 
shall  remain;  upon  sulphonating  a  sample  of  the  total  distillate,  the 
residue  shall  not  exceed  1  per  cent. 

For  applying  the  creosote  to  the  wood,  several  methods  are 
followed.  The  one  in  most  favor  for  paving  blocks  is  the  "pressure 
process",  which  essentially  consists  in:  (1)  steaming  the  wTood  for 
the  purpose  of  liquefying  the  sap  and  other  substances  contained  in 
the  interfibrous  spaces;  (2)  creating  a  vacuum  for  the  purpose  of 
removing  the  liquefied  substances;  (3)  injecting  the  creosote  under 
pressure. 

The  operation  is  performed  in  metal  cylinders  called  "retorts", 

6  or  more  feet  in  diameter  and  of  any  desired  length,  usually  about 
100  feet.     The  load  of  blocks,  called  a  "charge",  is  placed  upon 
metal  cars  called  "buggies"  and  is  run  into  the  retort  cylinder,  the 
ends  of  which  then  are  hermetically  closed  writh  "heads"  or  doors. 
Steam,  at  a  gage  pressure  varying  from  15  pounds  to  45  pounds  per 
square  inch,  is  admitted  to  the  retort  (in  some  plants  a  vacuum  is 
first  created)  and  the  pressure  maintained  for  several  hours.     When 
the  operator  considers  that  the  steaming  has  been  continued  a  suffi- 
cient length  of  time,  the  products  of  condensation  are  removed  from 
the  retort  through  a  blow-off  cock  in  the  bottom;  when  this  is  accom- 
plished an  air  exhaust,  or  vacuum  pump  is  put  in  operation,  and  a 
vacuum  of  from  20  inches  to  26  inches  is  created  and  maintained 
for  about  one  hour,  at  the  end  of  which  time  the  creosote  is  allowed 


HIGHWAY  CONSTRUCTION  149 

to  flow  into  the  retort  until  it  is  filled.  A  pressure  pump  then  is 
started  to  force  the  creosote  into  the  retort  until  the  pressure  reaches 
100  pounds  to  150  pounds  per  square  inch.  This  pressure  is  main- 
tained until  the  required  amount  of  creosote  has  been  injected  in 
the  wood,  then  the  surplus  is  drawn  off,  the  heads  opened,  and  the 
charge  withdrawn. 

The  amount  of  creosote  injected  into  the  wood  varies  from  10 
pounds  to  22  pounds  per  cubic  foot  of  wood.  The  amount  is  deter- 
mined primarily  by  measuring  the  tanks  and  is  verified  by  testing 
sample  blocks.  A  sample  block  is  bored  entirely  through  in  the 
direction  of  the  fiber  with  an  auger  1  inch  in  diameter,  the  hole 
being  located  midway  between  the  sides  and  about  J  the  length  of 
the  block  from  one  end.  The  borings  are  collected,  thoroughly 
mixed,  and  the  quantity  and  ratio  of  creosote  to  wood  in  the  borings 
determined  by  extracting  the  creosote  completely  with  carbon 
bisulphide. 

The  condition  of  the  wood  at  the  time  of  the  treatment,  is  prefer- 
ably dry  and  free  from  an  excess  of  water.  After  treatment,  and 
until  used,  the  blocks  during  dry  weather  should  be  sprinkled  fre- 
quently with  water  to  prevent  drying  and  cracking.  The  treated 
blocks  are  sometimes  subjected  to  tests  to  determine  the  resistance 
to  wear  when  saturated  with  water,  the  resistance  to  compression 
and  impact,  and  to  ascertain  the  amount  of  water  the  wood  will 
absorb. 

Laying  the  Blocks.  The  surface  of  the  concrete  foundation  is 
cleansed  from  dust  and  dirt  by  sweeping,  then  sprinkled  with  water. 
Upon  the  cleaned  surface  a  cushion  coat  is  formed,  by  spreading  a 
layer  of  sand  1  inch  thick,  Fig.  92,  or  a  mortar  composed  of  1  part 
Portland  cement  and  2  parts  sand,  mixed  with  sufficient  water  to 
form  a  stiff  paste  (the  practice  of  using  a  mixture  of  cement  and 
sand  slightly  moistened  with  water  produces  a  defective  pavement). 
The  blocks  are  set  upon  the  cushion  coat  with  the  fiber  vertical, 
Fig.  93,  in  straight,  parallel  courses  at  right  angles  to  the  axis  of  the 
street,  except  at  intersections  where  they  are  set  at  an  angle  of  45 
degrees  with  the  axis  of  the  street.  They  are  laid  so  as  to  have  the 
least  possible  width  of  joint  (wide  joints  hasten  the  destruction  of 
the  wood  by  permitting  the  fibres  to  broom  and  wear  under  traffic) . 
Blocks  in  adjoining  courses  break  joint  by  at  least  3  inches.  At  the 


150 


HIGHWAY  CONSTRUCTION 


Fig.  92.     Spreading  Sand  Foundation  for  Wood  Blocks  in  LaSalle  Street,  Chicago 
Courtesy  of  Engineering  News,  New  York  City 


Fig.  93.     Laying  Wood  Blocks  in  LaSalle  Street,  Chicago 
Courtesy  of  Engineering  News,  New  York  City 


HIGHWAY  CONSTRUCTION 


151 


Fig.  94.     Wood-Block  Pavement  Being  Hammered  and  Rolled,  Preparatory  to  Putting  in  Filler 
Courtesy  of  Engineering  News,  New  York  City 


Fig.  95.     Spreading  Sand  Filler  on  Wood-Block  Pavement 
Courtesy  of  Engineering  News,  New  York  City 


152  HIGHWAY  CONSTRUCTION 

curb  it  is  customary  to  place  one  or  two  rows  of  blocks  with  the 
length  parallel  to  the  curb  and  f  inch  therefrom. 

After  the  blocks  are  laid  they  are  brought  to  a  uniform  surface 
by  ramming  with  hand  rammers  or  rolling  with  a  light  steam  roller, 
Fig.  94.  When  laid  upon  a  mortar  cushion,  the  rolling  or  ramming 
must  be  completed  before  the  mortar  sets. 

In  some  cases  the  cushion  coat  is  omitted,  the  surface  of  the 
concrete  freed  from  dust  by  dry  sweeping  is  covered  with  a  thin 
coat  of  a  bituminous  cement  and  the  blocks  laid  directly  upon  it. 
Sometimes,  the  side  and  one  end  of  each  block,  when  it  is  about  to 
be  set  in  place,  are  dipped  in  the  same  bituminous  material  that  is 
used  to  cover  the  concrete,  the  blocks  are  placed  in  contact  and  the 
surface  is  covered  with  a  thin  coating  of  the  bituminous  material, 
this  being  covered  with  a  layer  of  sand  or  fine  gravel. 

After  the  blocks  have  been  brought  to  a  uniform  surface,  the 
joints  are  filled  with  either  fine  sand,  cement  grout,  or  a  bituminous 
cement,  Fig.  95.  When  sand  is  used,  it  should  be  fine  and  dry, 
spread  over  the  surface  of  the  pavement,  and  swept  about  until  the 
joints  are  filled.  Cement  grout  is  made  of  equal  parts  of  Portland 
cement  and  fine  sand  mixed  with  water  to  the  required  consistency. 
It  is  spread  over  the  surface  of  the  blocks  and  swept  into  the  joints 
until  they  are  filled.  The  surface  of  the  pavement  then  is  covered 
with  sand,  and  the  grout  is  allowed  to  set  for  about  seven  days 
before  traffic  is  admitted.  The  bituminous  filler  is  composed  of 
coal-tar  pitch,  asphalt,  or  combinations  of  these,  and  other  ingre- 
dients. The  filler  is  applied  hot  in  the  same  manner  as  described 
under  brick  pavement.  To  provide  for  the  expansion  of  the  blocks 
the  joint  next  the  curb  is  filled  with  bituminous  filler. 

Qualifications  of  Wood  Pavements.  Advantages.  The  advan- 
tages of  wood  pavement  may  be  stated  as  follows: 

(1)  It  affords  good  foothold  for  horses. 

(2)  It  offers  less  resistance  to  traction  than  stone,  and  slightly 
more  than  asphalt. 

(3)  It  suits  all  classes  of  traffic. 

(4)  It  may  be  used  on  grades  up  to  5  per  cent. 

(5)  It  is  moderately  durable. 

(6)  It  yields  no  mud  when  laid  upon  an  impervious  foundation. 

(7)  It  yields  but  little  dust. 


HIGHWAY  CONSTRUCTION  153 

(8)  It  is  moderate  in  first  cost. 

(9)  It  is  not  disagreeably  noisy. 

Defects.    The  principal  objections  to  wood  pavement  are : 

(1)  It  is  difficult  to  cleanse. 

(2)  Under  certain  conditions  of  the  atmosphere  it  becomes 
greasy  and  very  unsafe  for  horses.     This  may  be  remedied  by  cover- 
ing the  surface  with  a  thin  layer  of  fine  sand  or  gravel;  a  similar 
treatment  will  absorb  the  oil  which  exudes  during  warm  weather. 

(3)  It  is  not  easy  to  open  for  the  purpose  of  gaining  access  to 
underground  pipes,  it  being  necessary  to  remove  rather  a  large 
surface  for  this  purpose,  which  has  to  be  left  a  little  time  after  being 
repaired  before  traffic  again  is  allowed  upon  it. 

ASPHALT  PAVEMENTS 

Sheet=Asphalt  Pavement.  Sheet  asphalt  is  the  name  used  to 
describe  a  pavement  having  a  wearing  surface  composed  of  sand 
graded  in  predetermined  proportions,  of  a  fine  material  or  filler, 
and  of  asphalt  cement,  all  incorporated  by  mixing  in  a  mechanical 
mixer,  and  laid  upon  a  concrete  foundation,  the  surface  of  the  latter 
being  covered  with  a  thin  layer  of  bituminous  concrete  called  a 
"binder". 

Asphalt  Cement.  This  is  prepared  from  solid  bitumen,  refined 
and  fluxed  with  (1)  the  residuum  from  paraffine  petroleum;  (2)  the 
residuum  from  asphaltic  petroleum;  (3)  a  mixture  of  paraffine  and 
asphaltic  petroleum  residuums;  (4)  natural  malthas,  or  is  prepared 
from  (5)  solid  residual  bitumen  produced  in  the  distillation  of 
asphaltic  petroleums,  and  fluxed  with  residuum  oil  produced  from 
the  same  material. 

Refined  asphalt  is  that  freed  from  the  combined  water  and 
accompanying  inorganic  and  organic  matter.  By  comparatively 
simple  operations  the  several  varieties  of  asphalt  may  be  separated 
from  their  impurities.  Two  methods  are  employed  for  refining;  one 
using  steam  and  the  other  direct  fire.  In  both  methods  the  asphalt 
is  placed  in  tanks  and  slowly  heated  until  thoroughly  melted,  and 
during  the  melting  the  mass  is  agitated  by  a  current  of  either  air  or  dry 
steam.  The  method  of  using  steam  is  superior  to  the  fire  method. 
In  the  latter  method  there  always  is  danger  of  overheating,  in  addi- 
tion to  the  formation  of  coke  and  the  cracking  of  the  hydrocarbons. 


154  HIGHWAY  CONSTRUCTION 

The  varieties  of  asphalt  known  as  gilsonite  and  grahamite,  which 
are  practically  pure  bitumen,  do  not  require  refining,  but  they  are 
used  to  a  very  small  extent  in  paving. 

The  greater  part  of  the  solid  bitumen  used  for  paving  in  the 
United  States  is  obtained  from  the  \Yest  Indies  and  South  America. 
The  more  extensively  used  being  that  found  at  Trinidad,  W.  I.,  and 
at  Bermudez,  Venezuela.  The  asphalts  known  by  the  trade  names 
' 'California''  and  "texaco"  are  produced  by  refining  asphaltic  oils, 
and  may  or  may  not  require  to  be  fluxed. 

Fluxes  are  fluid  oils  and  tars  which  are  mixed  with  asphalt  to 
produce  a  desired  consistency.  The  refined  asphalt  is  melted  and 
the  flux  previously  heated  added  to  it,  in  the  proportion  required  to 
produce  the  desired  consistency.  The  mixture  of  asphalt  and  flux 
is  agitated  either  by  mechanical  means  or  by  a  blast  of  air  until  the 
materials  are  thoroughly  incorporated  and  the  desired  consistency 
is  obtained. 

Sand.  The  sand  should  be  siliceous  and  so  free  from  organic 
matter,  mica,  soft  grains,  and  other  impurities,  that  these  will  not 
amount  to  more  than  2  per  cent  of  its  volume. 

Fine  Material  or  Filler.  This  consists  of  any  sound  stone, 
usually  limestone  or  sand,  pulverized  to  such  fineness  that  the  whole 
will  pass  the  No.  50  sieve,  and  not  more  than  10  per  cent  will  be 
retained  on  the  No.  100  sieve,  and  at  least  70  per  cent  will  pass  the 
No.  200  sieve.  Portland  cement  sometimes  is  used  instead  of  the 
pulverized  stone. 

The  paving  composition  is  prepared  by  heating  the  ingredients 
separately  to  a  temperature  between  300  and  350  degrees  Fahren- 
heit, then  incorporating  them  by  mixing  in  a  mechanical  mixer. 
The  hot  sand  is  measured  into  the  mixer,  followed  by  the  hot  filler; 
these  two  materials  are  thoroughly  mixed  together,  and  the  hot 
cement  is  added  in  such  a  way  as  to  distribute  it  evenly  over  the 
mixed  sand  and  filler;  the  mixing  then  is  continued  until  the  materials 
form  a  uniform  and  homogeneous  mass,  with  the  grains  of  sand 
completely  covered  with  cement.  A  typical  mixture  is:  sand  100 
pounds;  filler  17.5  pounds;  bitumen  in  asphalt  cement  17.5  pounds. 

The  proportions  of  the  ingredients  in  the  paving  mixture  are 
not  constant,  but  vary  with  the  climate  of  the  place  where  the 
pavement  is  to  be  used,  the  character  of  the  sand,  and  the  amount 


HIGHWAY  CONSTRUCTION  155 

and  character  of  the  traffic  that  will  use  the  pavement.     The  ranges 
are  indicated  in  the  following  data: 


Data  for  Asphaltic  Paving 


Asphaltic  Paving  Mixture. 


CONSTITUENTS 
Asphalt  cement 
Sand 
Stone  dust 


PER  CENT 
12  to  15 

70  to  83 
5  to  15 


Weight  of  Material.  A  cubic  yard  of  the  prepared  material  weighs  about 
4500  pounds.  One  ton  of  refined  asphaltum  makes  about  2300  pounds  of 
asphalt  cement,  equal  to  about  3.4  cubic  yards  of  surface  material. 

Wearing  Surface  per  Cubic  Yard  of  Material. 

THICKNESS  AREA 

(inches)  (sq.  yd.) 

21  12 

2  IS 

11  27 

Laying  the  Pavement.    The  hot  paving  mixture  is  hauled  to 
the  street  and  dumped  at  a  place  outside  of  the  space  m  which  it 


spholttin. 

inder  l£  in. 
Concrete  5  in. 


Fig.  9f>.     Section  of  Aephalt  Pavement  Showing  Layers 

is  to  be  laid.  It  then  is  thrown  into  place  with  hot  shovels,  and 
spread  with  hot  rakes  uniformly  to  such  a  depth  as  will  give  the 
required  thickness  when  compacted.  The  finished  thickness 
varies  between  1|  inches  and  2  inches.  The  reduction  of  thickness 
by  compression  is  about  40  per  cent  generally.  Before  the  mixture 
is  spread,  the  surfaces  of  curbs  and  street  fittings  that  will  be  in 
contact  with  it  are  painted  with  hot  asphalt  cement. 

The  pavement  is  constructed  in  two  forms:     (1)  The  paving 
mixture  is  laid  directly  upon  the  surface  of  the  concrete  foundation; 


156  HIGHWAY  CONSTRUCTION 

(2)  the  surface  of  the  concrete  foundation  is  covered  with  a  coat 
of  asphaltic  concrete,  Fig.  96,  called  the  "binder  course",  the  object 
of  which  is  to  unite  more  securely  the  wearing  surface  to  the  foun- 
dation. This  it  does  by  containing  a  larger  percentage  of  cement, 
which,  if  put  in  the  surface  mixture,  would  render  it  too  soft.  The 
binder  is  composed  of  sound,  hard  stone  broken  to  pass  a  IJ-inch 
screen,  sand,  pulverized  stone,  and  asphalt  cement,  mixed  in  the 
desired  proportions.  A  typical  mixture  is:  stone  100  pounds; 
sand  40  pounds;  stone  dust  8  pounds;  bitumen  in  asphalt  cement 
8  pounds. 

The  paving  composition  is  compressed  by  means  of  rollers 
and  tamping  irons,  the  latter  being  heated  in  a  fire  contained  in  an 
iron  basket  mounted  on  wheels.  These  irons  are  used  for  tamping 
such  portions  as  are  inaccessible  to  the  roller,  namely,  gutters, 
around  manhole  heads,  etc. 

Two  rollers  are  sometimes  employed;  one,  weighing  5  to  6 
tons  and  of  narrow  tread,  is  used  to  give  the  first  compression; 
and  the  other,  weighing  about  10  tons  and  of  broad  tread,  is  used 
for  finishing.  The  rate  of  rolling  varies;  the  average  is  about  1 
hour  for  1000  square  yards  of  surface.  After  the  primary  com- 
pression, natural  hydraulic  cement,  or  any  impalpable  mineral 
matter,  is  sprinkled  over  the  surface,  to  prevent  the  adhesion  of 
the  material  to  the  roller  and  to  give  the  surface  a  more  pleasing 
appearance.  When  the  asphalt  is  laid  up  to  the  curb,  the  surface 
of  the  portion  forming  the  gutter  is  painted  with  a  coat  of  hot 
cement. 

Although  asphaltum  is  a  poor  conductor  of  heat,  and  the  cement 
retains  its  plasticity  for  several  hours,  occasions  may  and  do  arise 
through  which  the  composition  before  it  is  spread  has  cooled;  its 
condition  when  this  happens  is  analogous  to  hydraulic  cement  which 
has  taken  a  "set",  and  the  same  rules  which  apply  to  hydraulic 
cement  in  this  condition  should  be  respected  in  regard  to  asphaltic 
cement. 

If  the  temperature  of  the  air  at  the  time  of  hauling  is  below 
70  degrees  Fahrenheit  the  wagons  carrying  it  are  covered  with 
canvas  or  other  material  to  prevent  the  loss  of  heat.  The  tem- 
perature when  delivered  at  the  place  where  it  is  to  be  used  must 
not  be  less  than  280  degrees  Fahrenheit. 


HIGHWAY  CONSTRUCTION  157 

Two  methods  are  followed  in  laying  an  asphalt  pavement 
adjoining  street  railway  tracks:  (1)  a  course  of  granite-block  or 
brick  paving  is  laid  between  the  rail  and  the  edge  of  the  asphalt; 
(2)  the  asphalt  is  laid  directly  against  the  rail,  which,  if  its  tem- 
perature is  below  50  degrees  Fahrenheit,  is  heated  by  suitable 
apparatus  to  a  temperature  of  about  60  degrees  Fahrenheit 
immediately  before  the  asphalt  is  laid. 

Foundation.  A  solid,  unyielding  foundation  is  indispensable 
with  all  asphaltic  pavements,  because  asphalt  of  itself  has  no 
power  of  offering  resistance  to  the  action  of  traffic,  conse- 
quently it  nearly  always  is  placed  upon  a  bed  of  hydraulic- 
cement  concrete.  The  concrete  must  be  set  thoroughly  and 
its  surface  dry  before  the  asphalt  is  laid  upon  it;  if  not,  the 
water  will  be  sucked  up  and  converted  into  steam,  with  the 
result  that  coherence  of  the  asphaltic  mixture  is  prevented,  and, 
although  its  surface  may  be  smooth,  the  mass  is  really  honey- 
combed, so  that  as  soon  as  the  pavement  is  subjected  to  the  action 
of  traffic,  the  voids  or  fissures  formed  by  the  steam  appear  on 
the  surface,  and  the  whole  pavement  is  broken  up  quickly. 

Qualifications  of  Asphalt  Pavements.  Advantages.  These 
may  be  summed  up  as  follows : 

(1)  It  gives  easy  traction. 

(2)  It  is  comparatively  noiseless  under  traffic. 

(3)  It  is  impervious. 

(4)  It  is  easily  cleansed. 

(5)  It  produces  neither  mud  nor  dust. 

(6)  It  is  pleasing  to  the  -eye. 

(7)  It  suits  all  classes  of  traffic. 

(8)  There  is  neither  vibration   nor   concussion  in   traveling 
over  it. 

(9)  It  is  laid  expeditiously,  thereby  causing  little  inconvenience 
to  traffic. 

(10)  Openings  to  gain  access  to  underground  pipes  are  easily 
made. 

(11)  It  is  durable. 

(12)  It  is  repaired  easily. 
Defects.    These  are  as  follows: 

(1)     It  is  slippery  under  certain  conditions  of  the  atmosphere. 


158 


HIGHWAY  CONSTRUCTION 


The  American  asphalts  are  much  less  so  than  the  European,  on 
account  of  their  granular  texture  derived  from  the  sand.  The 
difference  is  very  noticeable;  the  European  are  as  smooth  as  glass, 


£ 

Granite  Curb\ 
Vitrified  BricK 

"a 

3 

•:".  Sand  ,   .;^. 

fi 

Asphalt 
Binder 

J 

r 

»0 

^S^A:^^^',  ||jj 

Sand  and  Cement 

I.;.'.  \i.....  -v; 

i 

r  v-  •<  .    -/  V-  ,"  r<rt>- 
•<  \?'  ^.'^  :  <}  77-  :  : 

^.Vl''.7- 

-I 
.1 


Fig.  97.     Section  of  Asphalt  Pavement  Showing  Use  of  Vitrified  Brick  to  Form  Gutter 

while  the  American  resemble  fine  sandpaper.  The  slipperiness 
can  be  decreased  by  heating  the  surface  of  the  pavement  with  a 
surface  heater,  then  applying  a  layer  of  coarse  sand  and  rolling 
it  into  the  surface. 

(2)  It  will  not  stand  constant  moisture,  and  will  disintegrate 
if  sprinkled  excessively. 

(3)  Under  extreme  heat  it  is  liable  to  become  so  soft  that  it 
will  roll  or  creep  under  traffic  and  present  a  wavy  surface;  and 
under  extreme  cold  there  is  danger  that  the  surface  will  crack  and 
become  friable. 

(4)  It  is  not  adapted  to  grades  steeper  than  2J  per  cent, 
although  it  is  in  use  on  grades  up  to  7.30  per  cent. 


Asphalt 


Brick    Gutter 


Curb 


Fig.  98.     Plan  of  Asphalt  Pavement  Showing  Use  of  Vitrified 
Brick  to  Form  Gutter 


(5)  Repairs  must  be  made  quickly,  for  the  material  has  little 
coherence,  and  if,  from  irregular  settlement  of  foundation  or  local 
violence,  a  break  occurs,  the  passing  wheels- rapidly  shear  off  the 
sides  of  the  hole,  and  it  soon  assumes  formidable  dimensions. 


HIGHWAY  CONSTRUCTION  159 

Although  pure  asphaltum  is  impervious  absolutely  and  insoluble 
in  either  fresh  or  salt  water,  yet  asphalt  pavements  in  the  continued 
presence  of  water  are  quickly  disintegrated.  Ordinary  rain  or 
daily  sprinkling  does  not  injure  them  when  they  are  allowed  to 
become  perfectly  dry  again.  The  damage  is  most  apparent  in 
gutters  and  adjacent  to  overflowing  drinking  fountains.  This 
defect  has  long  been  recognized,  and  various  measures  have  been 
taken  to  overcome  it,  or  at  least  reduce  it  to  a  minimum.  In  some 
cities,  ordinances  have  been  passed,  seeking  to  regulate  the  sprinkling 
of  the  streets;  and  in  many  places  the  gutters  are  laid  with  stone 
or  vitrified  brick,  Figs.  97  and  98,  while  in  others  the  asphalt  is 
laid  to  the  curb,  a  space  of  12  to  15  inches  along  the  curb  being 
covered  with  a  thin  coating  of  asphalt  cement. 

Failure  of  Asphalt  Pavement.  The  failure  of  asphalt  pavement 
is  due  to  any  one,  or  a  combination,  of  the  following  causes: 

(1)  Unsuitable  Materials.    The  asphalt  may  be  changed  so 
by  natural  causes  as  to  possess  little  or  no  cementing  power.    The 
fluxing  agent  may  form  only  a  mechanical  instead  of  a  chemical 
union  writh  the  asphalt,  or  its  character  may  be  such  as  to  render 
the  asphalt  brittle,  in  which  condition  it  easily  is  broken  up  under 
traffic.     The  sand  may  be  graded  improperly,  either  too  coarse  or 
too  fine,  or  contain  loam,  vegetable  matter,  or  clay. 

(2)  Improper  Manipulation.    The  crude  asphalt  may  have 
been  refined  at  too  high  a  temperature,  which  reduces  or  destroys 
the  cementing  property.    The  cement  may  be  of  improper  con- 
sistence,  of  insufficient  quantity,  or  inadequately  mixed.     If  the 
cement  is  too  hard,  the  pavement  will  have  a  tendency  to  crack 
during  cold  weather;  and  if  too  soft,  it  will  push  out  of  place  and 
form  wraves  under  traffic.     The  quantity  of  cement  needed  varies 
with  the  character  of  the  sand — a  fine  sand  requires  more  cement 
than  a  coarse  one,  and  the  proportion  of  cement  must  be  varied  to 
suit  the  sand.     When  the  ingredients  are  mixed  inadequately,  the 
cement  and  the  particles  of  sand  are  not  brought  into  intimate 
contact.     Free  oil  or  an  excess  of  asphalt  in  the  binder,  making 
it  too  rich,  is  liable  to  work  up  and  be  absorbed  by  the  wearing 
surface,  and  thus  cause  it  to  disintegrate.    The  mixture  may  be 
chilled  while  being  transported  from  the  plant  to  the  street.    There 
may  be  separation  of  the  cement  and  sand,  for  if  the  distance  from 


160  HIGHWAY  CONSTRUCTION 

the  plant  to  the  street  is  great  and  there  is  any  delay,  some  of  the 
cement  may  work  to  the  bottom  of  the  load,  and  when  it  is  dumped, 
there  will  be  fat  and  lean  spots,  both  of  which  are  injurious.  The 
paving  mixture  may  be  laid  upon  a  damp  or  dirty  foundation. 
There  may  be  inadequate  compression,  for  the  importance  of 
thorough  compaction  is  not  appreciated  always  and  this  portion 
of  the  work  is  slighted  frequently. 

(3)  Natural    Causes.    All    materials    in    nature    continually 
are  undergoing  change  due  to  the  action  of  the  elements,  and  to 
this  asphalt  is  not  an  exception.     Subjected  to  the  action  of  heat, 
all  bitumens   grow    harder,  and    when    the    maximum  degree  of 
hardness  is  attained,  natural  decay  sets  in  so  that  under  the  com- 
bined  action  of    the    elements,  the  material  gradually  rots   and 
disintegrates. 

(4)  Defective   Foundation.     By    unequal    settlement    a    weak 
foundation  will  cause  cracks  and  depressions  in  the  surface  which 
will  enlarge  speedily  under  traffic.     A  porous  foundation  will  per- 
mit the  ground  water  to  rise,  by  capillary  action,  to  the  underside 
of  the  wearing  surface,  where  by  freezing  it  will  cause  cracks  and 
thus  provide  access  for  surface  water;  non- watertight  connection 
between  curbs  and  street  fittings  also  furnishes  a  path  for  surface 
water  to  reach  the  underside  of  the  wearing  surface,  where  the 
presence  of  water  causes  rapid  decay. 

(5)  Other  Causes.    Illuminating  gas,   escaping  from  leaking 
pipes  under  the  pavement  causes  disintegration  of  the  asphalt. 
Contraction,  caused  by  the  decrease  in  cementing  power  through 
aging  of  the  asphalt,  results  in  cracks.     Due  to  an  excess  of  fluxing 
material,  there  may  be  rolling  and  waving  of  the  pavement  under 
traffic.     Injury  may  be  caused  by  fires  made  upon  the  pavement, 
or  by  oil  droppings  from  motor  vehicles. 

Sheet  asphalt  pavement  usually  is  constructed  under  a  contract 
that  provides  for  its  maintenance  during  a  period  of  years  (five 
or  ten)  by  the  contractor.  Such  a  contract  stipulates  that  the 
condition  of  the  pavement  at  the  expiration  of  the  maintenance 
or  guarantee  period  shall  be  as  follows:  Surface  free  from  depres- 
sions exceeding  f -inch  deep,  when  measured  between  any  two  points 
4  feet  apart  on  a  line  conforming  substantially  to  the  original  con- 
tour of  the  pavement.  Free  from  cracks.  Contain  no  disintegrated 


HIGHWAY  CONSTRUCTION  161 

material.  Thickness  not  reduced  more  than  f  inch.  Foundation 
free  from  cracks  and  settlement. 

Rock  Asphalt  Pavement.  This  is  the  name  applied  to  pave- 
ment made  from  the  limestones  and  sandstones  found  naturally 
impregnated  or  cemented  with  bitumen.  They  are  prepared  for 
use  by  crushing  and  heating,  and  are  used  in  their  natural  con- 
dition or  mixed  with  other  materials.  Deposits  are  found  in  many 
parts  of  the  United  States  and  Europe.  In  Europe,  rock  asphalt 
is  the  material  most  extensively  used  for  paving,  under  the  name 
"asphalte".  The  European  rock  asphalts  are  impregnated  very 
uniformly  with  from  7  per  cent  to  14  per  cent  of  asphalt,  and  readily 
compact  into  a  hard,  smooth  pavement  which  in  frosty  latitudes 
becomes  very  slippery.  The  American  rock  asphalts  are  impreg- 
nated irregularly  with  from  5  per  cent  to  30  per  cent  of  asphalt. 
Their  use  for  paving  is  limited,  chiefly  owing  to  the  cost  of  trans- 
portation. 

Asphalt  Blocks.  Formation.  Asphalt  paving  blocks  are  formed 
from  a  mixture  of  asphaltic  cement  and  crushed  stone  in  the  pro- 
portion of  8  or  12  per  cent  of  cement  to  88  or  92  per  cent  of  stone. 
The  materials  are  heated  to  a  temperature  of  about  300°  Fahrenheit, 
and  mixed  while  hot  in  a  suitable  vessel.  When  the  mixing  is 
complete,  the  material  is  placed  in  molds  and  subjected  to  heavy 
pressure,  after  which  the  blocks  are  cooled  suddenly  by  plunging 
into  cold  water.  The  usual  dimensions  of  the  blocks  are  4  inches 
wide,  3  inches  deep,  and  12  inches  long. 

Foundation.  The  blocks  usually  are  laid  upon  a  concrete 
foundation  with  a  cushion  coat  of  sand  about  J-inch  thick.  They 
are  laid  with  their  lengths  at  right  angles  to  the  axis  of  the  street, 
and  the  longitudinal  joints  should  be  broken  by  a  lap  of  at  least 
4  inches.  The  blocks  then  are  rammed  with  hand  hammers,  or 
are  rolled  with  a  light  steam  roller,  the  surface  being  covered 
with  clean,  fine  sand;  no  joint  filling  is  used,  as,  under  the  action 
of  the  sun  and  traffic,  the  blocks  soon  become  cemented. 

The  advantages  claimed  for  a  pavement  of  asphalt  blocks 
over  those  for  a  continuous  sheet  of  asphalt  are:  (1)  that  they  can 
be  made  at  a  factory  located  near  the  materials,  whence  they  can 
be  transported  to  the  place  where  they  are  to  be  used  and  can  be 
laid  by  ordinary  paviors,  whereas  sheet  pavements  require  special 


162 


HIGHWAY  CONSTRUCTION 


machinery  and  skilled  labor;  (2)  that  they  are  less  slippery,  owing 
to  the  joints  and  the  rougher  surface  due  to  the  use  of  crushed  stone. 


minr   mi 


Fig.  99.     Rake  and  Smoothing  Irons  Used  in  Laying  Asphalt  Pavement 
Courtesy  of  Barber  Asphalt  Paving  Company,  Philadelphia,  Pennsylvania 


Another  Form.    Another  form  of  asphalt  block,  known  as  the 

"Lueba"  block,  consists  of  a  block  8J  inches  long,  4J  inches  wide, 

^  and  4  inches  thick,  with  the  lower  3  inches 

'^EJ      JB  composed  of  Portland-cement  concrete  cov- 

%^H^^^       ered  with   1   inch   of   natural-rock   asphalt; 

|       the  two  materials  being  joined  under  heavy 

m^      hydraulic  pressure.     The  blocks  are  laid  on 

a   concrete  foundation    and   the    joints  be- 

^^|  lIpP  tween  them  are  filled  with  hydraulic-cement 

grout. 

Tools  Employed  in  Construction  of 
Asphalt  Pavements.  The  tools  used  in  lay- 
ing sheet-asphalt  pavements  comprise  hand 
rammers  iron,  rakes,  smoothing  irons,  Fig. 
99;  pouring  pots,  Fig.  100;  hand  rollers, 
either  with  or  without  a  fire  pot,  Fig.  101 ; 

Fig.  100.    Pouring  Pots  Used  3    .    . 

with  Asphalt  Pavements        and  steam  rollers,  with  or  without  pro  vision 

Courtesy  of  Barber  Asphalt  PI,'  .IP  n      T^        i  rvr»        rni 

Paving  company  for  heating  the  front  roll,  rig.  102.     Ihese 


HIGHWAY  CONSTRUCTION 


1G3 


rollers  are  different  in  construction,  appearance,  and  weight  from 
those  employed  for  compacting  broken  stone.  The  difference  is 
due  to  the  different  character  of  the  work  required. 


Fig.  101.     Hand  Rollers  Used  in  Laying  Asphalt  Pavements 
Courtesy  of  Barber  Asphalt  Paving  Company,   Philadelphia,   Pennsylvania 


Fig.  102.     Small  Road  Roller  Used  in  Laying  Asphalt  Pavements 
Courtesy  of  Barber  Asphalt  Paving  Company,  Philadelphia,  Pennsylvania 


The  principal  dimensions  of  the  5-ton  roller  are  as  follows; 

Front  roll  or  steering  wheel,  diameter 30  to  32  inches 

Real  roll  or  driving  wheel,  diameter 48  inches 

Front  roll,  width 40  inches 

Rear  roll,  width 40  inches 

Length,  extreme 14  feet 

Height,  extreme.  . 7  to    8  feet 

Water  capacity 80  to  100  gallons 

Coal  capacity 200  pounds 


164  HIGHWAY  CONSTRUCTION 


MISCELLANEOUS  PAVEMENTS 

Under  this  head  will  be  described  briefly  the  most  notable 
examples  of  pavements  devised  as  substitutes  for  the  recognized 
standard  types,  and  sometimes  used  where  good  materials  are  not 
available,  or  where  insufficient  funds  prevent  their  purchase,  and 
in  some  cases  for  the  purpose  of  utilizing  waste  products. 

Burnt  Clay.  In  the  Mississippi  Valley,  during  the  dry  season, 
the  clay  is  cut  from  the  roadway  to  a  depth  of  about  2  feet,  and 
piled  so  as  to  form  enclosures  about  15  feet  in  diameter  and  2  feet 
high.  After  remaining  so  for  about  ten  days  to  dry  out,  a  fire  is 
made  in  the  inclosure,  more  dry  clay  placed  on  top  and  the  burning 
proceeded  with.  The  burnt  clay  after  cooling,  is  relaid  upon  the 
road,  and  then,  being  of  a  thoroughly  porous  nature,  settles  into  a 
dry,  solid  layer. 

Straw.  Clay  roads  have  been  improved  by  shaping  and  harrow- 
ing the  road,  then  applying  a  layer  of  wheat  straw,  which  is 
moistened  with  water,  and  cut  and  mixed  with  the  clay  by  a  disk 
harrow.  More  straw  is  added  and  the  operation  repeated,  then  com- 
pacted with  a  steam  roller.  The  treatment  is  applied  twice  a  year. 

Oyster=Shell.  The  shells  are  spread  on  the  road  previously 
shaped  and  rolled.  They  crush  readily  and,  possessing  a  high 
cementing  quality,  bind  together  to  form  a  compact,  smooth  road 
surface,  but  owing  to  their  softness,  they  are  quickly  ground  to 
powder  which  is  carried  away  readily  by  wind  and  rain  water. 

Chert.  The  siliceous  material  found  overlying  the  red  sand- 
stone, which  forms  the  covering  of  the  red  hematite  iron  ore  in 
some  of  the  Southern  States,  is  used  for  both  street  and  road  paving. 
It  is  laid  directly  upon  the  earth  surface  or  upon  a  prepared  foun- 
dation, sprinkled,  and  compacted  in  the  same  manner  as  w^ater- 
bound  macadam. 

Slag.  The  slag  produced  in  the  manufacture  of  iron  and  steel 
is  used  in  various  ways  for  paving.  (1)  It  may  be  crushed  to 
the  desired  sizes  and  used  in  the  same  manner  as  broken  stone, 
laid  in  one  or  two  courses,  sprinkled  and  rolled.  In  some  cases, 
a  binder  composed  of  quicklime  is  used;  in  others,  a  waste  sulphite 
liquor  is  mixed  with  the  water  used  for  moistening  it  before  rolling; 
and  in  others,  it  is  mixed  with  coal-tar  or  other  bituminous  cement 


HIGHWAY  CONSTRUCTION  165 

and  formed  into  a  pavement  in  the  same  manner  as  bituminous 
macadam.  The  pavement  called  "tarmac",  a  large  amount  of 
which  has  been  used  in  England,  is  composed  of  slag,  coal  tar, 
rosin,  and  Portland  cement.  (2)  The  slag  may  be  formed  into 
blocks  by  casting  in  molds,  which  are  used  in  the  same  manner  as 
stone  blocks.  In  this  form  they  are  called  "scoria"  or  "slag"  blocks. 

Clinker.  Where  crematories  are  employed  for  the  destruction 
of  garbage  about  33  per  cent  of  the  material  remains  after  burning, 
in  the  form  of  clinker.  This  is  broken  up  and  ground  to  a  fine 
powder,  mixed  with  either  a  hydraulic  or  a  bituminous  cement,  and 
pressed  into  blocks  and  slabs. 

Petrolithic.  Petrolithic  paving  is  made  by  applying  a  bitu- 
minous oil  to  earth,  sand,  gravel,  clay,  or  loam  roads.  The  soil  is 
plowed  to  a  depth  of  at  least  6  inches,  pulverized  by  harrowing, 
and  sprinkled  with  water.  The  bituminous  oil  is  applied  in  one 
or  two  coats  at  the  rate  of  1  gallon  per  square  yard,  the  oil  and  soil 
are  mixed  and  compacted  by  a  roller  weighing  5000  pounds,  the 
surface  of  which  is  studded  with  spikes  having  a  flat  head  measuring 
2X3  inches,  and  on  which  account  it  is'  named  a  "sheep's-foot" 
roller.  In  operation,  the  spikes  or  feet  are  forced  into  the  loose 
soil  and  compress  or  pack  it  from  the  bottom  upwards.  After  a 
thorough  mixing  and  tamping,  the  surface  is  shaped  with  a  road 
grader  and  rolled  with  a  roller  of  the  ordinary  form. 

Kleinpflaster.  Kleinpflaster  is  the  name  given  to  a  stone  pave- 
ment used  in  Germany  for  exceptionally  heavy  traffic,  and  used  also 
in  England,  under  the  name  "durax".  It  is  made  of  3-inch  cubes 
of  hard  stone,  cut  by  machinery,  and  laid  in  small  segments  of 
circles.  The  stones  are  laid  as  close  as  possible  and  the  joints  are 
filled  with  hydraulic-cement  grout  or  bituminous  filler. 

Iron.  Several  experiments  have  been  made  with  iron  for 
paving,  but,  while  eminently  durable,  it  was  rough,  noisy,  and 
slippery,  and  its  use  either  alone  or  combined  with  other  materials 
has  been  abandoned. 

Trackways.  Formed  of  stone  slabs,  brick,  concrete  blocks, 
steel,  and  other  materials,  trackways  have  been  constructed  at 
various  times  for  the  purpose  of  reducing  the  resistance  to  traction. 
Their  use  on  an  extensive  scale,  however,  has  been  abandoned 
except  in  Italy,  Spain,  and  Germany. 


166  HIGHWAY  CONSTRUCTION 

National  Pavement.  National  pavement  is  composed  of  pul- 
verized clay,  loam,  or  ordinary  soil,  heated  and  mixed  with  liquid 
bitumen.  The  mixture  is  spread  to  a  depth  of  2  or  3  inches  upon 
the  surface  of  the  compacted  and  drained  natural  soil  and  is  com- 
pressed by  a  power  roller. 

Fibered  Asphalt  Pavement.  Fibered  asphalt  pavement  is  com- 
posed of  wood  fiber,  obtained  as  a  waste  product  from  the  process 
of  extracting  tannin  and  asphalt.  The  fiber  is  heated  and  mixed 
with  a  predetermined  quantity  of  asphalt.  The  hot  mixture  is  run 
into  molds  forming  small  blocks  which  are  shipped  to  the  place  of 
use.  The  blocks  are  there  heated  to  a  temperature  of  275°  F.  in  a 
traveling  heater  that  moves  along  the  roadway  and  from  which  the 
hot  mixture  emerges  in  a  continuous  stream  18  inches  wide  and  is 
deposited  on  the  previously  prepared  foundation  to  a  depth  of  4 
inches.  After  spreading,  it  is  compressed  to  a  thickness  of  2  inches 
with  a  power  roller. 

Westrumite.  Westrumite  is  an  asphalt  cement  temporarily 
liquefied  by  emulsification.  It  is  mixed  cold  with  broken  stone  in 
an  ordinary  concrete  mixer,  spread  on  the  foundation,  and  com- 
pacted by  rolling.  The  evaporation  of  the  vehicle  leaves  the  asphalt 
cement  as  the  binder. 

MISCELLANEOUS  STREET  WORK 
FOOTPATHS 

A  footpath  or  walk  is  simply  a  road  under  another  name — a 
road  for  pedestrians  instead  of  one  for  horses  and  vehicles.  The 
only  difference  that  exists  is  in  the  degree  of  service  required;  but 
the  conditions  of  construction  that  render  a  road  well  adapted  to 
its  object  are  very  much  the  same  as  those  required  for  a  walk. 

The  effects  of  heavy  loads  such  as  traverse  carriage-ways  are 
not  felt  upon  footpaths;  but  the  destructive  action  of  wrater  and 
frost  is  the  same  in  either  case,  and  the  treatment  to  counteract 
or  resist  these  elements  as  far  as  practicable,  and  to  produce  per- 
manency, must  be  the  controlling  idea  in  each  case,  and  should 
be  carried  out  upon  a  common  principle.  It  is  not  less  essential 
that  a  walk  should  be  well  adapted  to  its  object  than  that  a  road 
should  be;  and  it  is  annoying  to  find  it  impassable  or  insecure  and 


HIGHWAY  CONSTRUCTION  167 

in  want  of  repair  when  it  is  needed  for  convenience  or  pleasure. 
In  point  of  economy,  there  is  the  same  advantage  in  constructing 
a  footway  skilfully  and  durably  as  there  is  in  the  case  of  a  road. 

Width.  The  width  of  footwalks  (exclusive  of  the  space  occu- 
pied by  projections  and  shade  trees)  should  be  ample  to  accommodate 
comfortably  the  number  of  people  using  them.  In  streets  devoted 
entirely  to  commercial  purposes,  the  clear  width  should  be  at  least 
one-third  the  width  of  the  carriage-way;  in  residential  and  suburban 
streets,  a  very  pleasing  result  can  be  obtained  by  making  the  walk 
one-half  the  width  of  the  roadway,  and  by  devoting  the  greater 
part  to  grass  and  shade  trees. 

Cross  Slope.  The  surface  of  footpaths  must  be  sloped  so  that 
the  surface  water  will  readily  flow  to  the  gutters.  This  slope  need 
not  be  very  great;  J  inch  per  foot  will  be  sufficient.  A  greater  slope 
with  a  thin  coating  of  ice  upon  it,  becomes  dangerous  to  pedestrians. 

Foundation.  As  in  the  case  of  roadways,  so  with  footpaths, 
the  foundation  is  of  primary  importance.  Whatever  material  may 
be  used  for  the  surface,  if  the  foundation  is  weak  and  yielding,  the 
surface  wrill  settle  irregularly  and  become  extremely  objectionable, 
if  not  dangerous,  to  pedestrians. 

Surface.    The  requirements  of  a  good  covering  for  sidewalks  are : 

(1)  It  must  be  smooth  but  not  slippery. 

(2)  It  must  absorb  the  minimum  amount  of  water,  so  that  it 
may  dry  rapidly  after  rain. 

(3)  It  must  not  be  abrased  easily. 

(4)  It  must  be  of  uniform  quality  throughout,  so  that  it  may 
wear  evenly. 

(5)  It  must  neither  scale  nor  flake. 

(6)  Its  texture  must  be  such  that  dust  will  not  adhere  to  it. 

(7)  It  must  be  durable. 

Materials.  The  materials  used  for  footpaths  are  as  follows: 
stone,  natural  and  artificial;  wood;  asphalt;  brick;  tar  concrete;  and 
gravel. 

Stones.  Of  the  natural  stones,  sandstone  (bluestone)  and 
granite  are  employed  extensively.  The  bluestone,  when  well  laid, 
forms  an  excellent  paving  material.  It  is  of  compact  texture, 
absorbs  water  to  a  very  limited  extent,  and  hence  soon  dries  after 
rain;  it  has  sufficient  hardness  to  resist  abrasion,  and  wears  well 


168  HIGHWAY  CONSTRUCTION 

without  becoming  excessively  slippery.  Granite,  although  exceed- 
ingly durable,  wears  very  slippery,  and  its  surface  has  to  be  rough- 
ened frequently. 

Slabs,  of  whatever  stone,  must  be  of  equal  thickness  throughout 
their  entire  area;  the  edges  must  be  dressed  true  to  the  square 
for  the  whole  thickness  (edges  must  not  be  left  feathered  as  shown 


Fig.  103.     Faulty  Joint  in  Stone  Sidewalk 

in  Fig.  103) ;  and  the  slabs  must  be  bedded  solidly  on  the  foundation 
and  the  joints  filled  with  cement  mortar.  Badly  set  or  faultily 
dressed  flagstones  are  very  unpleasant  to  walk  over,  especially 
in  rainy  weather;  the  unevenness  causes  pedestrians  to  stumble, 
and  rocking  stones  squirt  dirty  water  over  their  clothes. 

Wood.  Wood  has  been  used  largely  in  the  form  of  planks; 
it  is  cheap  in  first  cost,  but  proves  very  expensive  from  the  fact  that 
it  lasts  but  a  comparatively  short  time  and  requires  constant  repair 
to  keep  it  from  becoming  dangerous. 

Asphalt.  Asphalt  forms  an  excellent  footway  pavement;  it 
is  durable  and  does  not  wear  slippery. 

Brick.  Brick  of  suitable  quality,  well  and  carefully  laid  on  a 
concrete  foundation,  makes  an  excellent  footway  pavement  for 
residential  and  suburban  streets  of  large  cities,  and  also  for  the  main 
streets  of  smaller  towns.  The  bricks  should  be  good  qualities  of 
paving  brick  (ordinary  building  brick  are  unsuitable,  as  they  soon 
wear  out  and  are  broken  easily).  The  bricks  should  be  laid  in 
parallel  rows  on  their  edges,  with  their  lengths  at  right  angles  to 
the  axis  of  the  path. 

Concrete.  Concrete  or  artificial  stone  is  used  extensively  as 
a  footway  paving  material.  Its  manufacture  is  the  subject  of 
several  patents,  and  numerous  kinds  are  to  be  had  in  the  market. 
When  manufactured  of  first-class  materials  and  laid  in  a  sub- 
stantial manner,  with  proper  provision  against  the  action  of 
frost,  artificial  stone  forms  a  durable,  agreeable,  and  inexpensive 
pavement. 


HIGHWAY  CONSTRUCTION  169 

Concrete  walks  are  formed  in  one  or  two  courses.  In  one-course 
work,  the  concrete  is  laid  to  a  depth  of  4  inches  and  tamped  until 
sufficient  mortar  flushes  to  the  surface  to  permit  the  forming  of  a 
smooth  surface.  In  two-course  work,  the  concrete  for  the  base 
is  spread  and  tamped  to  a  depth  of  3  inches,  the  top  or  surface 
course  is  spread  upon  the  base  before  the  latter  has  begun  to  set. 
The  top  course  has  a  thickness  of  about  1  inch,  and  it  is  tamped 
and  its  surface  is  brought  to  the  required  plane  by  a  straightedge 
and  by  troweling.  Expansion  is  provided  for  by  transverse  joints 
extending  the  full  depth  of  the  concrete.  The  joints  are  placed 
4  feet  apart  and  are  formed  by  placing  across  the  side  forms  a  J-inch 
thick  metal  dividing  strip,  which  is  removed  before  the  cement 
sets  so  that  the  edges  of  the  joint  may  be  smoothed  and  rounded 
with  a  suitable  tool. 

The  area  to  be  covered  by  the  walk  is  excavated  to  a  minimum 
depth  of  8  inches,  or  to  such  greater  depth  as  the  nature  of  the  ground 
may  require  to  secure  a  solid  foundation.  The  surface  of  the  ground 
so  exposed  is  compacted  by  ramming,  and  a  drainage  course  is 
formed  of  broken  stone,  gravel,  or  steam-plant  cinders,  thoroughly 
compacted  by  ramming,  and  its  surface  is  brought  to  a  plane  parallel 
to  and  4  inches  below  the  finished  surface  of  the  concrete.  In 
some  situations  it  may  be  necessary  to  connect  the  drainage  course 
with  the  sewers,  street  drains,  or  side  ditches,  for  the  purpose  of 
furnishing  an  outlet  for  standing  water;  this  is  done  by  the  use  of 
3-inch  drain  pipe  placed  where  required. 

The  forms  of  steel  or  wood  should  be  made  substantially,  and 
left  in  place  until  the  concrete  is  set  hard. 

Concrete  walks  fail  from  the  use  of  improper  materials  and 
defective  workmanship,  insufficient  expansion  joints,  heaving  and 
cracking  by  frost,  due  to  imperfect  drainage,  displacement  and 
cracks,  due  to  settlement  of  the  drainage  course — this  latter 
being  frequent  when  cinders  are  used,  as  in  time  they  are  liable 
to  decompose  and  shrink  in  volume  and  thus  allow  the  con- 
crete to  settle.  In  two-course  \vork  failure  may  be  in  respect 
to  flaking  of  the  surface  by  the  action  of  water  and  frost  entering 
between  and  separating  the  courses.  The  concrete  should  not  be 
laid  during  freezing  weather,  nor  should  frozen  materials  be  used 
in  the  work. 


170 


HIGHWAY  CONSTRUCTION 


CURBSTONES  AND  GUTTERS 

Curbstones.  Curbstones  are  employed  for  the  outer  side  of 
footways,  to  sustain  the  covering  and  to  form  ^the  gutter.  Their 
upper  edges  are  set  flush  with  the  footwalk  pavement,  so  that  the 
water  can  flow  over  them  into  the  gutters 


Fig.  104.     Typical  Section  Showing  Stone  Curb  Eight  Inches  Thick 

The  disturbing  forces  which  the  curb  has  to  resist  are:  (1)  The 
pressure  of  the  earth  behind  it,  which  is  frequently  augmented  by 
piles  of  merchandise,  building  materials,  etc.  This  pressure  tends 
to  overturn  it,  break  it  transversely,  or  move  it  bodily  on  its  base. 
(2)  The  pressure  due  to  the  expansion  of  freezing  earth  behind 


Fig.  105.     Typical  Section  Showing  Stone  Curb  Five  Inches  Thick 

and  beneath  the  curb.  This  force  is  most  frequent  where  the  side- 
walk is  sodded  partly  and  the  ground  accordingly  is  moist.  Suc- 
cessive freezing  and  thawing  of  the  earth  behind  the  curb  will  occasion 
a  succession  of  thrusts  forward,  which,  if  the  curb  be  of  faulty 
design,  will  cause  it  to  incline  several  degrees  from  the  vertical. 


HIGHWAY  CONSTRUCTION  171 

(3)  The  concussions  and  abrasions  caused  by  traffic.  To  withstand 
the  destructive  effect  of  wheels,  curbs  are  faced  with  iron;  and  a 
concrete  curb  with  a  rounded  edge  of  steel  has  been  patented  and 
used  to  some  extent.  Fires  built  in  the  gutters  deface  and  seriously 
injure  the  curb.  Posts  and  trees  set  too  near  the  curb,  tend  to 
break,  displace,  and  destroy  it. 

The  use  of  drain  tiles  under  the  curb  is  a  subject  of  much  dif- 
ference of  opinion  among  engineers.  Where  the  subsoil  contains 
water  naturally,  or  is  likely  to  receive  it  from  outside  the  curb  lines, 
the  use  of  drains  is  of  decided  benefit;  but  great  care  must  be  exer- 
cised in  jointing  the  draintiles,  lest  the  soil  shall  be  loosened  and 
removed,  causing  the  curb  to  drop  out  of  alignment. 

The  materials  employed  for  curbing  are  the  natural  stones — 
as  granite,  sandstone  (bluestone),  etc.;  artificial  stone — fire  clay, 
and  cast  iron. 

The  dimensions  of  curbstones  vary  considerably  in  different 
localities  and  according  to  the  width  of  the  footpaths;  the  wider  the 
path,  the  wider  should  be  the  curb.  However,  it  should  be  never 
less  than  8  inches  deep,  nor  narrower  than  4  inches.  Depth  is 
necessary  to  prevent  the  curb's  turning  over  toward  the  gutter.  It 
never  should  be  in  smaller  lengths  than  3  feet.  The  top  surface 
should  be  beveled  off  to  conform  to  the  slope  of  the  footpath.  The 
front  face  should  be  hammer-dressed  for  a  depth  of  about  6  inches, 
in  order  that  there  may  be  a  smooth  surface  visible  against  the 
gutter.  The  back  for  3  inches  from  the  top  also  should  be  dressed, 
so  that  the  flagging  or  other  paving  may  butt  fair  against  it.  The 
end  joints  should  be  cut  a  true  square  the  full  thickness  of  the  stone 
at  the  top,  and  so  much  below  the  top  as  will  be  exposed;  the  remain- 
ing portion  of  the  depth  and  bottom  should  be  squared  roughly, 
and  the  bottom  should  be  fairly  parallel  to  the  top.  (See  Figs.  104 
and  105.) 

Combination  Curb  and  Gutter.  Concrete  curb  and  gutter 
combined  is  constructed  by  placing  the  concrete  in  suitable  forms. 
The  concrete  should  be  handled  so  as  to  prevent  the  separation 
of  the  stone  and  mortar,  and  when  placed  should  be  tamped  well 
to  bring  the  mortar  to  the  surface  and  make  complete  contact  with 
the  forms.  The  corner  formed  by  the  top  and  face  surfaces  is 
rounded  to  a  radius  of  about  1J  inches;  sometimes  this  corner  is 


172 


HIGHWAY  CONSTRUCTION 


formed  of  a  steel  bar  put  in  place  before  the  concrete  is  laid  and 
anchored  by  metal  strips  spaced  about  3  feet  apart.  Expansion 
joints  are  formed  at  distances  of  10  or  12  feet.  The  remarks  made 
under  concrete  walks  regarding  foundation,  drainage,  failure,  etc., 
apply  also  to  concrete  curbs. 

STREET  CLEANING 

The  cleaning  of  streets  is  practiced  for  the  purpose  of  protecting 
the  health  of  the  neighboring  residents  and  for  the  comfort  of  the 
users.  It  is  of  comparatively  recent  development,  and  is  ren- 
dered possible  only  by  the  use  of  hard  pavements.  The  materials 


Fig.  106.     Typical  Machine  Street  Sweeper 
Courtesy  of  Acme  Road  Machinery  Company,  Frankfort,  New  York 

to  be  removed  from  the  streets  consist  of  animal  droppings,  material 
worn  from  the  pavement,  materials  dropped  from  vehicles,  waste 
from  building  construction,  miscellaneous  materials  swept  from 
houses,  stores,  and  factories,  and  the  accumulation  of  snow  during 
winter. 

Cleaning  Methods.  The  local  conditions  and  character  of  the 
traffic  and  pavement  determine  the  methods  to  be  employed  and 
the  intervals  for  cleaning  the  streets.  The  methods  employed  are: 
sweeping,  either  by  hand  or  by  machine  brooms;  and  flushing  with 
water — the  work  being  performed  either  during  the  day  or  the 
night,  by  large  gangs  at  night,  and  by  means  of  a  patrol  system 
during  the  day.  Fig.  106  shows  one  of  the  machine  sweepers  used. 


HIGHWAY  CONSTRUCTION 

TABLE  XV 
Rate  and  Cost  of  Street  Cleaning 


173 


PAVEMENT 

APPROXIMATE 
SURFACE  SWEPT 
PER  MAN 
(sq.  yds.  per  hr.) 

APPROXIMATE 
DIRT  FROM  DAILY 
SWEEPING 
(cu.  yds.  per 
1000  sq.  yds.) 

AVERAGE  COST 
PER  EACH 
CLEANING 
(cents  per  sq. 

yd.) 

(Wet) 

(Dry) 

(Mia.) 

(Max.) 

Asphalt 

1000 

1200 

.007 

.040 

.0030 

Granite-block 

750 

1000 

.015 

.024 

.0050 

Macadam  (water-bound) 

700 

.100 

.350 

.0106 

Wood 

.070 

.200 

.0070 

Brick 

.0034 

In  the  hand-cleaning  method  by  day  patrol,  each  man  is  fur- 
nished with  a  push  broom,  shovel,  and  can  carrier  in  which  to  place 
the  refuse,  and  has  a  certain  section  of  street  to  clean  each  day. 
The  day  patrol  sometimes  is  supplemented  by  a  large  gang  working 
during  the  night.  When  machine  brooms  are  employed  they  usually 
are  operated  at  night  and  are  supplemented  by  the  patrol  system 
during  the  day.  As  to  which  is  the  most  economical,  it  will  depend 
upon  the  cost  of  labor  and  the  condition  of  the  pavements;  on  pave- 
ments covered  with  ruts  and  depressions  machine  brooms  are 
ineffective. 

The  approximate  costs  of  the  various  methods  of  street  cleaning 
per  1000  sq.  yds.  are: 

Sweeping  (hand) $0.281 

Sweeping  (machine) 0 . 317 

Flushing  (hand-hose) 0 . 319 

Flushing  (machine) 0 . 721 

The  average  cost  of  supervision  varies  from  .011  cent  to  34 
cents  per  mile. 

The  amount  of  surface  cleaned  by  a  machine  broom  depends 
upon  the  width  of  the  broom,  the  power  of  the  horses  or  other 
motive  power,  gradient,  and  condition  of  the  surface.  The  wider 
the  broom  the  less  will  be  the  cost.  The  average  speed  of  travel 
is  about  1J  miles  per  hour. 

In  Table  XV  are  indicated  the  amount  of  surface  which  an 
average  man  will  sweep  per  hour,  depending  upon  the  condition 
of  the  pavement — dry,  wet,  or  muddy;  relative  amount  of  dirt 


HIGHWAY  CONSTRUCTION  175 

produced  by  the  different  pavements,  if  swept  daily;  and  the  average 
cost  of  cleaning  different  pavements. 

Removal  of  Snow.  The  methods  employed  for  keeping  roads 
and  streets  passable  during  the  period  of  snowfall  varies  according 
to  the  climatic  conditions.  In  localities  subject  to  heavy  falls 
of  snow,  and  continuous  low  temperature  that  retards  the  melting 
of  the  snow  until  spring,  two  methods  are  followed:  (1)  a  narrow 
track  is  opened  by  a  snowplow,  through  the  center  of  the  road, 
the  snow  being  formed  into  long,  narrow  heaps  on  each  side; 
(2)  the  snow  is  not  removed,  but  is  compacted  by  rolling  with  a 
light-weight  wood  or  metal  roller,  Fig.  107.  In  localities  having 
light  falls  and  in  the  larger  cities,  the  snow  is  pushed  by  plows  or 
rotary  brooms  toward  the  gutters  from  where  it  is  loaded  into 
vehicles,  hauled  to  a  natural  waterway  and  dumped,  or  in  the 
absence  of  this  it  is  placed  in  vacant  lots  and  in  some  cases  it  is 
disposed  of  by  dumping  into  the  sewers  through  the  manholes, 
but  this  must  be  done  carefully,  as  there  is  liability  of  choking  the 
sewer  by  the  snow's  consolidating.  Light  falls  may  be  disposed 
of  by  the  application  of  a  stream  of  water  to  the  surface  of  the 
street  thereby  washing  the  snow  into  the  sewer.  Many  machines 
have  been  devised  for  melting  the  snow  by  the  application  of  steam, 
hot  air,  etc.,  but  none  of  them  have  been  successful  economically. 
In  some  cities  the  snow  is  melted  by  an  application  of  rock  salt 
which  produces  a  thawing  action  when  mixed  with  the  snow  by 
the  traffic,  the  slushy  mixture  so  formed  is  swept  to  the  gutters 
by  machine  brooms  and  washed  into  the  sewers  by  a  stream  of 
water  from  the  hydrants.  Objection  is  made  to  this  method  on 
account  of  the  intense  cold  produced  and  its  injurious  effect  upon 
the  feet  of  pedestrians  and  on  the  hoofs  of  horses. 

In  order  to  cause  the  minimum  of  inconvenience  to  traffic  it 
is  necessary  that  the  snow  be  removed  from  the  streets  as  quickly 
as  possible,  therefore,  it  is  customary,  before  the  arrival  of  winter, 
to  lay  out  the  method  and  organization  required  and  to  make  arrange- 
ments for  the  quick  mobilization  of  the  force  needed  for  its  removal. 
To  accomplish  this  the  city  is  divided  into  districts,  in  each  of  which 
there  is  established  a  headquarters  and  depot  stocked  with  the 
necessary  tools  to  execute  the  work  in  that  district,  and  to  which 
the  laborers  report  when  the  snow  commences  to  fall. 


176  HIGHWAY  CONSTRUCTION 

Street  Sprinkling.  Streets  and  roads  are  sprinkled  with  water 
for  the  purpose  of  abating  dust  and  cooling  the  air.  While  water- 
bound  macadam  and  earth  surfaces  must  be  sprinkled  to  abate 
the  dust,  a  stone-block,  brick,  asphalt,  or  wood  pavement  will  not 
require  sprinkling  if  thoroughly  cleansed  and  kept  clean.  On 
unclean  and  badly  maintained  pavements,  sprinkling  with  water 
as  usually  performed  converts  the  fine  dust  into  a  slime  which  renders 
all  smooth  pavements  slippery,  and  in  warm  weather  it  becomes  a 
prolific  breeding  place  for  disease  germs,  it  clings  to  the  feet  and 
clothing  of  pedestrians,  and,  with  its  accompanying  germs,  is  carried 
into  buildings  and  dwellings. 

The  average  cost  of  sprinkling  per  square  yard  is  $0.009. 

The  systems  followed  for  executing  the  work  of  street  cleaning, 
snow  removal,  and  sprinkling  are:  (1)  by  contract  where  the  con- 
tractor furnishes  all  the  tools  and  labor;  (2)  by  contract  for  the 
labor  only,  the  city  furnishing  the  tools  and  machinery ;  (3)  by  the 
city,  with  its  own  staff  and  machinery. 

SELECTING  THE  PAVEMENT 

The  problem  of  selecting  the  best  pavement  for  any  particular 
case  is  a  local  one,  not  only  for  each  city,  but  also  for  each  of  the  various 
parts  into  which  the  city  is  imperceptibly  divided;  and  it  involves  so 
many  elements  that  the  nicest  balancing  of  the  relative  values  for  each 
kind  of  pavement  is  required  in  arriving  at  a  correct  conclusion. 

In  some  localities,  the  proximity  of  one  or  more  paving  materials 
determines  the  character  of  the  pavement;  while  in  other  cases  a 
careful  investigation  may  be  required  in  order  to  select  the  most 
suitable  material.  Local  conditions  always  should  be  considered; 
hence  it  is  not  possible  to  lay  down  any  fixed  rule  as  to  what  material 
makes  the  best  pavement. 

Qualifications.  The  qualities  essential  to  a  good  pavement 
may  be  stated  as  follows : 

(1)  It  should  be  impervious. 

(2)  It  should  afford  good  foothold  for  horses  and  adhesion 
for  motor  vehicles. 

(3)  It  should  be  hard  and  durable,  so  as  to  resist  wear  and 
disintegration. 

(4)  It  should  be  adapted  to  every  grade. 


HIGHWAY  CONSTRUCTION  177 

(5)  It  should  suit  every  class  of  traffic. 

(6)  It  should  offer  the  minimum  resistance  to  traction. 

(7)  It  should  be  noiseless. 

(8)  It  should  yield  neither  dust  nor  mud. 

(9)  It  should  be  cleaned  easily. 

(10)  It  should  be  cheap. 

Interests  Affected.  Of  the  above  requirements,  numbers  (2),« 
(4),  (5),  and  (6)  affect  the  traffic  and  determine  the  cost  of  haulage 
by  the  limitations  of  loads,  speed,  and  wear  and  tear  of  horses  and 
vehicles.  If  the  surface  is  tough  or  the  foothold  bad,  the  weight 
of  the  load  a  horse  can  draw  is  decreased,  thus  necessitating  the 
making  of  more  trips  or  the  employment  of  more  horses  and  vehicles 
to  move  a  given  weight.  A  defective  surface  necessitates  a  reduction 
in  the  speed  of  movement  and  a  consequent  loss  of  time;  it  increases 
the  wear  of  horses,  thus  decreasing  their  life  service  and  lessening 
the  value  of  their  current  services ;  it  also  increases  the  cost  of  main- 
taining vehicles  and  harness. 

Requirements,  numbers  (7),  (8),  and  (9),  affect  the  occupiers 
of  adjacent  premises,  who  suffer  from  the  effect  of  dust  and  noise; 
they  also  affect  the  owners  of  said  premises,  whose  income  from 
rents  is  diminished  where  these  disadvantages  exist.  Numbers 
(3)  and  (10)  affect  the  taxpayers  alone — first,  as  to  the  length  of 
time  during  which  the  covering  remains  serviceable;  and  second, 
as  to  the  amount  of  the  annual  repairs.  Number  (1)  affects  the 
adjacent  occupiers  principally  on  hygienic  grounds.  Numbers  (7) 
and  (8)  affect  both  traffic  and  occupiers. 

Problem  Involved  in  Selection.  The  problem  involved  in  the 
selection  of  the  most  suitable  pavement  consists  of  the  following 
factors:  (1)  adaptability;  (2)  desirability;  (3)  serviceability;  (4)  com- 
parative safety;  (5)  durability;  (6)  cost. 

Adaptability.  The  best  pavement  for  any  given  roadway  will 
depend  altogether  on  local  circumstances.  Pavements  must  be 
adapted  to  the  class  of  traffic  that  will  use  them.  The  pavement 
suitable  for  a  road  through  an  agricultural  district  will  not  be  suit- 
able for  the  streets  of  a  manufacturing  center ;  nor  will  the  covering 
suitable  for  heavy  traffic  be  suitable  for  a  pleasure  drive  or  for  a 
residential  district. 

General  experience  indicates  the  relative  fitness  of  the  several 


178  HIGHWAY  CONSTRUCTION 

TABLE  XVI 
Resistance  to  Traction  on  Different  Pavements 


TRACTIVE  RESISTANCE 

Lb.  per  Ton 

Fraction  of  the  Load 

Sheet-asphalt 
Brick 

30  to    70 
15  to    40 

eV  to  -3\, 

Cobblestone 

50  to  100 

40  tO  2]o 

Stone-block 

30  to    80 

eV  to  & 

Rectangular  wood-block 

30  to    50 

eV  to  4*0 

Round  wood-block 

40  to    80 

s1,)  to  2'5 

materials  as  follows:  for  country  roads,  suburban  streets,  and  pleasure 
drives — broken  stone;  for  streets  having  heavy  and  constant  traffic 
— rectangular  blocks  of  stone,  laid  on  a  concrete  foundation,  with 
the  joints  filled  with  bituminous  or  Portland-cement  grout;  for 
streets  devoted  to  retail  trade,  and  where  comparative  noiselessness 
is  essential — asphalt,  wood,  or  brick.  More  recent  experience 
indicates  that  concrete,  when  properly  laid  and  reinforced  at  neces- 
sary points,  may  be  employed  to  advantage  for  any  pavement, 
both  as  base  and  as  wearing  surface. 

Desirability.  The  desirability  of  a  pavement  is  its  possession 
of  qualities  which  make  it  satisfactory  to  the  people  using  and 
seeing  it.  Between  two  pavements  alike  in  cost  and  durability, 
people  will  have  preferences  arising  from  the  condition  of  their 
health,  personal  prejudices,  and  various  other  intangible  influences, 
causing  them  to  select  one  rather  than  the  other  in  their  respective 
streets.  Such  selections  often  are  made  against  the  demonstrated 
economies  of  the  case,  and  usually  in  ignorance  of  them.  Whenever 
one  kind  of  pavement  is  more  economical  and  satisfactory  to  use 
than  is  any  other,  there  should  not  be  any  difference  of  opinion 
about  securing  it,  either  as  a  new  pavement  or  in  the  replacement 
of  an  old  one. 

The  economic  desirability  of  pavements  is  governed  by  the 
ease  of  movement  over  them,  and  is  measured  by  the  number  of 
horses  or  pounds  of  tractive  force  required  to  move  over  them  a 
given  weight — usually  1  ton.  The  resistance  offered  to  traction 
by  different  pavements  is  shown  in  Table  XVI. 


HIGHWAY  CONSTRUCTION  179 

Serviceability.  The  serviceability  of  a  pavement  is  its  quality 
of  fitness  for  use.  This  quality  is  measured  by  the  expense  caused 
to  the  traffic  using  it — namely,  the  wear  and  tear  of  horses  and 
vehicles,  loss  of  time,  etc.  No  statistics  are  available  from  which 
to  deduce  the  actual  cost  of  wear  and  tear. 

The  serviceability  of  any  pavement  in  great  measure  depends 
upon  the  amount  of  foothold  afforded  by  it  to  the  horses — provided, 
however,  that  its  surface  be  not  so  rough  as  to  absorb  too  large  a 
percentage  of  the  tractive  energy  required  to  move  a  given  load 
over  it.  Cobblestones  afford  excellent  foothold,  and  for  this  reason 
are  largely  employed  by  horse-car  companies  for  paving  between 
the  rails;  but  the  resistance  of  their  surface  to  motion  requires 
the  expenditure  of  about  40  pounds  of  tractive  energy  to  move 
a  load  of  1  ton.  Asphalt  affords  the  least  foothold ;  but  the  tractive 
force  required  to  overcome  the  resistance  it  offers  to  motion  is 
only  about  30  pounds  per  ton. 

Comparative  Safety.  The  comparison  of  pavements  in  respect 
to  safety,  is  the  average  distance  traveled  before  a  horse  falls.  The 
materials  affording  the  best  foothold  for  horses  are  as  follows, 
stated  in  the  order  of  their  merit: 

(1)  Earth,  dry  and  compact. 

(2)  Gravel. 

(3)  Broken  stone  (macadam). 

(4)  Wood. 

(5)  Sandstone  and  brick. 

(6)  Asphalt. 

(7)  Granite  blocks. 

,  ^  Durability.  The  durability  of  pavement  is  that  quality  which 
determines  the  length  of  time  during  which  it  is  serviceable,  and 
does  not  relate  to  the  length  of  time  it  has  been  down.  The  only 
measure  of  durability  of  a  pavement  is  the  amount  of  traffic  tonnage 
it  will  bear  before  it  becomes  so  worn  that  the  cost  of  replacing  it 
is  less  than  the  expense  incurred  by  its  use. 

As  a  pavement  is  a  construction,  it  necessarily  follows  that  there 
is  a  vast  difference  between  the  durability  of  the  pavement  and  the 
durability  of  the  materials  of  which  it  is  made.  Iron  is  eminently 
durable;  but,  as  a  paving  material,  it  is  a  failure. 

The  durability  of  a  paving  material  will  vary  considerably  with 


180 


HIGHWAY  CONSTRUCTION 


TABLE  XVII 
/     Terms  of  Life  of  Various  Pavements 


MATERIALS 

TERMS  OF  LIFE 
(Years) 

Granite-block 

12  to  30 

Sandstone 

6  to  12 

Asphalt 
Wood 

10  to  14 

7  to  15 

Limestone 

1  to    3 

Brick 

5  to  15 

Macadam 

5  to    ? 

the  condition  of  cleanliness  observed.  One  inch  of  overlying  dirt 
will  protect  the  pavement  most  effectually  from  abrasion,  and 
prolong  its  life  indefinitely.  But  the  dirt  is  expensive;  it  injures 
apparel  and  merchandise,  and  is  the  cause  of  sickness  and  discom- 
fort. In  the  comparison  of  different  pavements,  no  traffic  should 
be  credited  to  the  dirty  one.  The  life  or  durability  of  different 
pavements  under  like  conditions  of  traffic  and  maintenance,  may 
be  taken  as  shown  in  Table  XVII. 

Cost.  First  cost  or  the  cost  of  construction,  is  largely  controlled 
by  the  locality  of  the  place,  its  proximity  to  the  particular  material 
used,  and  the  character  of  the  foundation.  The  question  of  cost 
is  the  one  which  usually  interests  taxpayers,  and  is  problably  the 
greatest  stumblingblock  in  the  attainment  of  good  roadways.  The  first 
cost  usually  is  charged  against  the  property  abutting  on  the  highway 
to  be  improved.  The  result  is  that  the  average  property  owner 
always  is  anxious  for  a  pavement  that  costs  little,  because  he  must 
pay  for  it,  not  caring  for  the  fact  that  cheap  pavements  soon  wear 
out  and  become  a  source  of  endless  annoyance  and  additional  expense. 
Thus  false  ideas  of  economy  usually  have  stood,  and  undoubtedly 
always  will  stand  to  some  extent,  in  the  way  of  realizing  that  the 
best  is  the  cheapest. 

The  pavement  which  has  cost  the  most  is  not  always  the  best; 
nor  is  that  which  cost  the  least  the  cheapest;  the  one  which  is  truly 
the  cheapest  is  the  one  which  makes  the  most  profitable  returns 
in  proportion  to  the  amount  expended  upon  it.  No  doubt  there 
is  a  limit  of  cost  to  go  beyond  which  would  produce  no  practical 
benefit;  but  it  always  will  be  found  more  economical  to  spend  enough 


HIGHWAY  CONSTRUCTION  181 

to  secure  the  best  results,  and  this  always  will  cost  less  in  the  long 
run.  One  dollar  well  spent  is  many  times  more  effective  than  one- 
half  of  the  amount  injudiciously  expended  in  the  hopeless  effort 
to  reach  sufficiently  good  results.  The  cheaper  work  may  look  as 
well  as  the  more  expensive,  for  a  time,  but  very  soon  may  have 
to  be  done  over  again. 

Economic  Benefit.  The  economic  benefit  of  a  good  roadway 
is  comprised  in  the  following:  its  cheaper  maintenance,  the  greater 
facility  it  offers  for  traveling,  thus  reducing  the  cost  of  transporta- 
tion; the  lower  cost  of  repairs  to  vehicles,  and  less  wear  of  horses, 
thus  increasing  their  term  of  serviceability  and  enhancing  the  value 
of  their  present  service ;  the  saving  of  time ;  and  the  ease  and  comfort 
afforded  to  those  using  the  roadway. 

Relative  Economies.  The  relative  economies  of  pavements — 
whether  of  the  same  kind  in  different  condition,  or  of  different 
kinds  in  like  good  condition — are  determined  sufficiently  by  summing 
their  cost  under  the  following  headings  of  account: 

(1)  Annual  interest  upon  first  cost  and  sinking  fund. 

(2)  Annual  expense  for  maintenance. 

(3)  Annual  cost  for  cleaning  and  sprinkling. 

(4)  Annual  cost  for  service  and  use. 

(5)  Annual  cost  for  consequential  damages. 

Interest  on  First  Cost.  The  first  cost  of  a  pavement,  like  any 
other  permanent  investment,  is  measurable  for  purposes  of  comparison 
by  the  amount  of  annual  interest  on  the  sum  expended.  Thus, 
assuming  the  worth  of  money  to  be  4  per  cent,  a  pavement  costing 
$4  per  square  yard  entails  an  annual  interest  loss  or  tax  of  $0.16 
per  square  yard. 

Cost  of  Maintenance.  Under  this  head  must  be  included  all 
outlays  for  repairs  and  renewals  which  are  made  from  the  time 
when  the  pavement  is  new  and  at  its  best  to  a  time  subsequent, 
when,  by  any  treatment,  it  is  put  again  in  equally  good  condition. 
The  gross  sum  so  derived,  divided  by  the  number  of  years  which 
elapse  between  the  two  dates,  gives  an  annual  average  cost  for 
maintenance. 

Maintenance  means  the  keeping  of  the  pavement  in  a  condition 
practically  as  good  as  when  first  laid.  The  cost  will  vary  con- 
siderably depending  not  only  upon  the  material  and  the  manner  in 


182  HIGHWAY  CONSTRUCTION 

which  it  is  constructed,  but  upon  the  condition  of  cleanliness 
observed,  and  the  quantity  and  quality  of  the  traffic  using  the 
pavement. 

The  prevailing  opinion  that  no  pavement  is  a  good  one  unless, 
when  once  laid,  it  will  take  care  of  itself,  is  erroneous;  there  is  no 
such  pavement.  All  pavements  are  being  worn  constantly  by 
traffic  and  by  the  action  of  the  atmosphere;  and  if  any  defects 
which  appear  are  not  repaired  quickly,  the  pavements  soon  become 
unsatisfactory  and  are  destroyed.  To  keep  them  in  good  repair, 
incessant  attention  is  necessary,  and  is  consistent  with  economy. 
Yet  claims  are  made  that  particular  pavements  cost  little  or  nothing 
for  repairs,  simply  because  repairs  in  these  cases  are  not  made, 
while  any  one  can  see  the  need  of  them. 

Cost  of  Cleaning  and  Sprinkling.  Any  pavement,  to  be  con- 
sidered as  properly  cared  for,  must  be  kept  dustless  and  clean. 
While  circumstances  legitimately  determine  in  many  cases  that 
streets  must  be  cleaned  at  daily,  weekly,  or  semiweekly  intervals, 
the  only  admissible  condition  for  the  purpose  of  analysis  of  street 
expenses  must  be  that  of  like  requirements  in  both  or  all  cases 
subjected  to  comparison. 

The  cleaning  of  pavements,  as  regards  both  efficiency  and  cost, 
depends  (1)  upon  the  character  of  the  surface;  (2)  upon  the  nature 
of  the  materials  of  which  the  pavements  are  composed.  Block 
pavements  present  the  greatest  difficulty;  the  joints  can  never 
be  perfectly  cleaned.  The  order  of  merit  as  regards  facility  of  cleans- 
ing, is:  (1)  asphalt,  (2)  concrete,  (3)  brick,  (4)  stone,  (5)  wood, 
(6)  macadam. 

Cost  of  Service  and  Use.  The  annual  cost  for  service  is  made 
up  by  combining  several  items  of  cost  incidental  to  the  use  of  the 
pavement  for  traffic — for  instance,  the  limitation  of  the  speed  of 
movement,  as  in  cases  where  a  bad  pavement  causes  slow  driving 
and  consequent  loss  of  time ;  or  cases  where  the  condition  of  a  pave- 
ment limits  the  weight  of  the  load  which  a  horse  can  haul,  and  so 
compels  the  making  of  more  trips  or  the  employment  of  more 
horses  and  vehicles;  or  cases  where  conditions  are  such  as  to 
cause  greater  wear  and  tear  of  vehicles,  of  equipment,  and  of 
horses.  If  a  vehicle  is  run  1500  miles  in  a  year,  and  its  main- 
tenance cost  $30  a  year,  then  the  cost  of  its  maintenance  per 


HIGHWAY  CONSTRUCTION  183 

mile  traveled  is  2  cents.  If  the  value  of  a  team's  time  is,  say 
$1  for  the  legitimate  time  taken  in  going  1  mile  with  a  load, 
and  in  consequence  of  bad  roads  it  takes  double  that  time,  then 
the  cost  to  traffic  from  having  to  use  that  mile  of  bad  roadway  is 
$1  for  each  load.  The  same  reasoning  applies  to  circumstances 
where  the  weight  of  the  load  has  to  be  reduced  so  as  to  neces- 
sitate the  making  of  more  than  one  trip.  Again,  bad  pavements 
lessen  not  only  the  life  service  of  horses,  but  also  the  value  of  their 
current  service. 

Cost  for  Consequential  Damages.  The  determination  of  conse- 
quential damages  arising  from  the  use  of  defective  or  unsuitable 
pavements,  involves  the  consideration  of  a  wide  array  of  diverse 
circumstances.  Rough-surfaced  pavements,  when  in  their  best 
condition,  afford  a  lodgment  for  organic  matter  composed  largely 
of  the  urine  and  excrement  of  the  animals  employed  upon  the  road- 
way. In  warm  and  damp  weather,  these  matters  undergo  putre- 
factive fermentation,  and  become  the  most  efficient  agency  for 
generating  and  disseminating  noxious  vapors  and  disease  germs, 
now  recognized  as  the  cause  of  a  large  part  of  the  ills  afflicting 
mankind.  Pavements  formed  of  porous  materials  are  objectionable 
on  the  same,  if  not  even  stronger,  grounds. 

Pavements  productive  of  dust  and  mud  are  objectionable, 
and  especially  so  on  streets  devoted  to  retail  trade.  If  this  par- 
ticular disadvantage  be  appraised  at  so  small  a  sum  per  lineal  foot 
of  frontage  as-  $1.50  per  month,  or  6  cents  per  day,  it  exceeds  the 
cost  of  the  best  quality  of  pavement  free  from  these  disadvantages. 

Rough-surfaced  pavements  are  noisy  under  traffic  and  insuffer- 
able to  nervous  invalids,  and  much  nervous  sickness  is  attributable 
to  them.  To  all  persons  interested  in  nervous  invalids,  this  damage 
from  noisy  pavements  is  rated  as  being  far  greater  than  would 
be  the  cost  of  substituting  the  best  quality  of  noiseless  pavement; 
but  there  are,  under  many  circumstances,  specific  financial  losses, 
measurable  in  dollars  and  cents,  dependent  upon  the  use  of  rough, 
noisy  pavements.  They  reduce  the  rental  value  of  buildings  and 
offices  situated  upon  streets  so  paved — offices  devoted  to  pursuits 
wherein  exhausting  brain  work  is  required.  In  such  locations, 
quietness  is  almost  indispensable,  and  no  question  about  the  cost 
of  a  noiseless  pavement  weighs  against  its  possession. 


184 


HIGHWAY  CONSTRUCTION 


TABLE  XVIII 
Comparative  Rank  of  Pavements 


CHARACTERISTICS 

VARIETY 

Asphalt  (sheet) 

Asphalt  (block) 

Concrete 

Value 

Macadam  (bituminous) 

Qualities 

(per 

Macadam  (water-bound) 

cent) 

Brick 

Granite 

Sandstone 

Wood) 

Low  tractive  resistance 

20 

20.C 

19.0 

18.0 

19.0 

ll.G 

18.0 

12.C 

14.0 

20.0 

Service  on  grades 

10 

3.0 

3.0 

7.0 

4.0 

8.0 

9.0 

10.0 

10.0 

2.0 

Non-slipperiness 

5 

1.5 

2.5 

4.0 

2.5 

4.5 

3.5 

3.5 

5.0 

2.0 

Favorableness  to  travel 

5 

5.0 

4.5 

3.5 

4.0 

4.5 

3.5 

3.5 

4.0 

4.5 

Sanitariness 

10 

10.0 

9.0 

7.0 

8.0 

3.0 

8.0 

6.0 

7.0 

9.0 

Noiselessness 

3 

2.5 

2.5 

2.0 

2.5 

2.5 

1.5 

1.0 

1.5 

3.0 

Minimum  dust 

3 

2.5 

2.5 

2.0 

2.0 

1.0 

2.0 

1.5 

2.0 

2.0 

Ease  of  cleaning 

5 

5.0 

5.0 

3.5 

4.0 

1.0 

3.5 

1.5 

1.5 

5.0 

Acceptability 

4 

3.5 

3.5 

2.5 

3.0 

1.5 

2.5 

2.0 

2.5 

4.0 

Durability 

15 

7.5 

8.5 

6.0 

3.0 

1.5 

10.0 

15.0 

14.0 

11.5 

Ease  of  maintenance 

5 

3.5 

4.0 

3.0 

3.0 

2.5 

4.0 

4.5 

5.0 

5.0 

Cheapness  (first  cost) 

10 

4.5 

4.0 

5.0 

7.5 

10.0 

4.0 

3.0 

3.5 

3.0 

Low  annual  cost 

5 

1.5 

2.5 

3.0 

3.5 

1.0 

4.5 

5.0 

5.0 

5.0 

Totals.  .  .  . 

100 

70.0 

70.5 

66.5 

66.0 

52.0 

74.0 

68.5 

75.0 

76.0 

Approximate  first    cost 
(dollars  per  sq.  yd.)  

2.30 

2.65 

1.85 

1.35 

1.00 

2.65 

3.25 

3.00 

3.45 

When  an  investigator  has  done  the  best  he  can  to  determine 
such  a  summary  of  costs  of  a  pavement,  he  may  divide  the  amount 
of  annual  tonnage  of  the  street  traffic  by  the  amount  of  annual 
costs,  and  know  what  number  of  tons  of  traffic  are  borne  for  each 
cent  of  the  average  annual  cost,  which  is  the  crucial  test  for  any 
comparison,  as  follows: 

(1)  Annual  interest  upon  first  cost  and  sinking  fund $ 

(2)  Average  annual  expense  for  maintenance  and  renewal .  . 

(3)  Annual  cost  for  custody  (sprinkling  and  cleaning) ...... 

(4)  Annual  cost  for  service  and  use 

(5)  Annual  cost  for  consequential  damages 

Amount  of  average  annual  cost 

Annual  tonnage  of  traffic 

Tons  of  traffic  for  each  cent  of  cost 

Gross  Cost  of  Pavements.  Since  the  cost  of  a  pavement 
depends  upon  the  material  of  which  it  is  formed,  the  width  of  the 


HIGHWAY  CONSTRUCTION  185 

roadway,  the  extent  and  nature  of  the  traffic,  and  the  condition 
of  repair  and  cleanliness  in  which  it  is  maintained,  it  follows  that 
in  no  two  streets  is  the  endurance  or  the  cost  the  same,  and  the 
difference  between  the  highest  and  lowest  periods  qf  endurance 
and  amount  of  cost  is  very  considerable. 

Comparative  Rank  of  Pavements.  In  Table  XVIII  is  given 
the  rank  of  the  various  pavements  in  percentage,  prorated  from 
the  values  assigned  in  the  first  column  to  the  desired  qualities. 
The  pavement  ranking  first  in  any  given  quality  is  given  the 
full  value  for  that  quality,  the  others  grading  down  from  this 
value  according  to  the  extent  to  which  they  possess  the  desired 
quality.  An  examination  of  the  table  shows  macadam  to  be  the 
cheapest;  least  durable,  and  most  difficult  to  maintain  and  cleanse; 
rather  favorable  to  travel;  comparatively  low  in  sanitariness;  and 
high  in  annual  cost.  While  the  table  may  be  used  as  an  aid  in 
determining  the  most  suitable  pavement  according  to  the  factors 
that  are  susceptible  of  a  numerical  value,  the  values  assigned  must 
be  modified  by  local  conditions;  first  cost  will  necessarily  vary  in 
different  localities,  and  certain  factors  will  be  more  important  in 
one  locality  than  another. 

Specifications.  A  specification  or  detailed  description  of  the 
various  works  to  be  carried  out  always  is  attached  to  a  contract, 
and  is  prepared  before  estimates  are  called  for.  The  prominent 
points  that  are  essential  to  the  production  of  a  specification  that 
will  fulfill  its  purpose  properly,  are:  (1)  description  of  the  work; 
(2)  extent  of  the  work;  (3)  quality  of  the  materials;  (4)  tests  for 
the  materials;  (5)  delivery  of  the  materials;  (6)  character  of  the 
workmanship;  (7)  manner  of  executing  the  work. 

Attention  to  these  points  and  a  clear  and  accurate  description 
of  each  detail  (leaving  nothing  to  be  imagined)  not  only  will 
contribute  materially  to  the  rapid  and  efficient  execution  of  the 
work,  but  will  avoid  any  future  misunderstanding.  Every  item 
of  the  work  should  be  allotted  a  separate  clause,  for  confusion 
must  ensue  when  a  single  clause  includes  descriptions  of  several 
matters. 

As  a  rule  it  is  undesirable  to  insert  in  specifications  any  dimen- 
sions or  weights  that  can  be  shown  on  the  drawings.  However, 
when  it  is  necessary  to  insert  them,  words  should  be  used  instead 


186  HIGHWAY  CONSTRUCTION 

of  numerals;  the  use  of  numerals,  and  particularly  decimal  numbers, 
should  be  avoided,  as  there  is  a  risk  of  having  them  set  up  incorrectly 
by  the  typesetter  and  overlooked  in  the  proofreading.  When  a 
numeral  is  used  it  should  be  followed  by  the  word  or  words  indicat- 
ing the  numeral,  placing  the  numeral  in  parenthesis. 

Brevity,  so  far  as  it  is  consistent  with  completeness,  should 
prevail,  but  the  word  "et  cetera"  should  be  excluded  rigidly,  and 
the  matters  covered  by  it  should  be  defined  clearly.  Neither  should 
important  points  of  the  work  be  dismissed  with  the  direction  that 
"the  wrork  shall  be  done  to  the  satisfaction  of  the  engineer".  A 
direction  of  this  kind  usually  implies  that  the  engineer  does  not 
know  what  he  wants,  and  therefore  leaves  the  matter  to  the  superior 
knowledge  of  the  contractor — an  attitude  not  very  creditable  to 
the  former.  The  only  really  legitimate  use  of  this  phrase  is  in  a 
general  clause  referring  to  the  whole  of  the  work. 

The  specifying  of  impracticable  sizes  of  materials  must  be 
avoided  as  it  causes  unnecessary  discussion  and  frequently  leads 
to  a  charge  for  "extras". 

A  clause  or  phrase  permitting  the  furnishing  of  alternative 
materials  or  workmanship  should  be  excluded,  because  such  per- 
mission affords  ground  for  dispute  and  difference  of  opinion.  On 
the  other  hand,  specifying  that  certain  articles  manufactured  by 
a  particular  firm  shall  be  used  should  be  avoided,  as  it  suggests 
unfairness  on  the  part  of  the  engineer,  and  may  create  the  idea 
that  his  selection  is  not  without  profit  to  himself. 

With  regard  to  the  actual  methods  of  carrying  out  the  work, 
the  contractor  should  not  be  tied  to  any  particular  means  of  effecting 
the  required  end,  unless  special  circumstances  require  it,  for,  pro- 
vided the  materials  and  workmanship  are  satisfactory,  it  is  better 
to  allow  the  contractor  to  use  his  own  discretion  as  to  the  manner 
of  producing  the  required  result. 

While  the  standard  and  proper  tests  for  the  materials  always 
should  be  stipulated,  yet  if  they  are  carried  to  an  extreme  degree, 
as  frequently  happens,  they  defeat  their  own  object.  When  it 
becomes  impossible  to  carry  out  certain  unreasonable  demands, 
the  alternative  is  to  evade  them  as  much  as  possible;  and  it  must 
be  borne  in  mind  that  the  more  stringent  the  demand,  the  greater 
the  difficulty  in  enforcing  it. 


HIGHWAY  CONSTRUCTION  187 

Contracts.  A  good,  clear,  and  comprehensive  contract  is  a 
difficult  thing  to  write,  but  it  should  be  "common  sense"  from 
beginning  to  end,  and  should  be  the  joint  production  of  both  engi- 
neering and  legal  ability,  neither  sacrificing  the  one  feature  to  the 
other. 

The  stipulations  of  the  contract  form  the  legal  part  of  the 
document  and  are  distinct  from  the  technical  description  of  the 
work  to  be  done.  The  essential  points  are:  (1-)  time  of  commence- 
ment; (2)  time  of  completion;  (3)  manner  and  times  of  payment; 
(4)  prices  for  which  the  work  is  to  be  performed;  (5)  measurements; 
(6)  damages  for  noncompletion;  (7)  protection  of  persons  and 
property  during  the  prosecution  of  work;  (8)  such  special  stip- 
ulations as  may  be  required  for  the  particular  work  that  is  being 
contracted  for. 

It  should  be  borne  in  mind  that  the  contract  and  specifications 
when  duly  signed  by  the  parties  interested,  is  a  legal  document, 
which  must  be  produced  in  court  in  the  event  of  a  dispute  arising, 
therefore,  it  is  of  the  utmost  importance  that  it  be  written  clearly 
in  simple  language,  the  clauses  being  arranged  in  logical  sequence, 
and  the  descriptions  made  exact  and  complete  without  being  need- 
lessly verbose. 

High-sounding  phrases,  and  duplication  of  statements  or  infor- 
mation, should  be  avoided  as  tending  to  confusion.  Specifications 
are  seldom  judged  by  literary  standards  of  excellence,  therefore, 
words  may  be  repeated  again  and  again  if  they  express  the  meaning 
of  the  writer  more  clearly  and  forcibly  than  an  alternative  would  do. 

In  the  case  of  a  lengthy  contract  and  specification,  a  complete 
index  with  the  clause  and  page  numbers  will  be  found  an  aid  to 
finding  quickly  any  required  subject;  cross  references  may  some- 
times be  introduced  with  advantage. 


INDEX. 


INDEX 


Asphalt  pavements 153 

asphalt  blocks 161 

failure ~ 159 

foundation 157 

laying 155 

qualifications 157 

rock  asphalt 161 

sheet-asphalt 153 

tools  employed  in  constructing  __ __ 162 

Axle  friction -  - 11 

B 

Bituminous-macadam 99 

amiesite 103 

asphaltic  petroleums 105 

bitulithic. . 103 

bituminous  materials,  definitions  of 104 

bituminous  materials,  tests  for 106 

cement,  bituminous 104 

construction 99 

features 99 

rock  asphalt 103 

Brick  pavements 133 

brick,  qualifications  of  good 133 

brick-pavement,  qualifications  of 137 

fillings,  joint 139 

foundation 137 

hand  tools  used 145 

heaters,  gravel 146 

laying,  manner  of 138 

machine,  concrete-mixing 145 

sand  cushion 137 

test _ 136 

Broken-stone  road 85 

compacting  stone 93 

construction 85 

macadam  and  telford  roads,  suppression  of  dust  on • 95 

quality 88 

rock,  testing 89 

shape  and  size  of  stonea 92 

species  of  stone _ 91 

spreading  stone 92 

thickness  of  stones.  _  92 


INDEX 


^  PAGE 

City  streets  and  highways _ 113 

arrangement 113 

asphalt 153 

brick •_ 168 

cleaning 172 

drainage 119 

foundations ^ 121 

grades 114 

pavement,  miscellaneous 164 

selecting 176 

stone-block 123 

street  work,  miscellaneous 166 

transverse  contour  or  crown 118 

width .' 113 

wood 147 

City  streets,  foundations  for 121 

concrete 122 

Concrete  pavements 106 

bituminous  surface,  with 108 

block  or  cube 108 

construction 106 

joints,  expansion 108 

materials 1 107 

reinforced-concrete 108 

Country  roads  and  boulevards 1 

location 1 12 

maintenance  and  improvement 110 

methods,  preliminary  construction 35 

nature-soil  roads 74 

vehicles,  resistance  to  movement  of 1 

Country  roads  and  boulevards,  drainage  of 38 

ends  from  weather,  protection  of 42 

fall,  for 40 

gutters .  43 

hillside  roads 45 

location 39 

materials 40 

outlets 43 

road  gutters,  inner  and  outer , 45 

side  ditches 43 

silt 42 

soils,  nature  of 39 

springs  in  cuttings,  treatment  of 44 

water  breaks 46 

Culverts 46 

arch i 53 

box..  52 


INDEX  3 

Culverts  (Continued)  PAGE 

design,  factors  in 46 

earthenware  pipe 49 

functions i__  46 

iron  pipe 51 

short  span  bridges  used  as 53 

E 

Earthwork 55 

classification 57 

cuts  and  fills,  balancing 55 

embankments,  methods  of  forming 58 

prosecution  of 58 

shrinkage  in 57 

slopes,  side _ 55 

tools 60 

G 

Grade  problems 33 

establishing  a  grade 34 

level  stretches 34 

minimum '__  33 

undulating 33 

Gravel  roads 83 

laying  gravel 84 

preparation  of  gravel 83 

repair 85 

L 

Location  of  roads 12 

instruments 14 

object  of 13 

points  to  consider 13 

reconnoissance 13 

selection,  final 22 

elements  entering  into 22 

grade  problems 33 

gradient 31 

location,  final 30 

mountain  roads,  treatment  of 26 

profile,  construction  of ._ 31 

repose,  angle  of 32 

roads,  alignment  of 27 

typical  cases,  treatment  of 23 

survey,  preliminary 15 

bridge  site 22 

features • ^._ 15 

map ,.  20 

memoir _ 22 

topography _ 15 


4  INDEX 

M  PAGE 

Miscellaneous  pavement 164 

burnt  clay 164 

chert 164 

clinker 165 

iron 165 

kleinpflaster 165 

oyster-shell 164 

petrolithic 165 

slag 164 

straw 164 

trackways 165 

Miscellaneous  street  work 166 

curbstones  and  gutters 170 

footpaths 166 

foundation 167 

materials ,. 167 

slope,  cross 167 

surface 167 

width _' - 167 

Mountain  roads,  treatment  of 26 

N 

Nature-soil  roads 74 

earth 74 

sand 77 

sand  and  gravel  soils,  application  of  oil  to 78 

sand-clay 77 

R 

Roads,  maintenance  and  improvement  of 110 

broken-stone __  109 

improvement  of  existing  roads 110 

systems 110 

traffic  census _ 111 

Roads  with  special  coverings 79 

foundations 79 

materials 79 

preparation  __: 80 

thickness _--  79 

types 81 

road  covering,  elements  of 79 

surfaces,  wearing 82 

bituminous-macadam 99 

broken-stone 85 

classification 82 

concrete 106 

function..                                         --  82 


INDEX  5 

Roads  with  special  coverings  PAGE 

surfaces,  wearing  (Continued) 

gravel - — ---     83 

thickness 82 

S 

Selecting  pavement -  176 

benefit,  economic 181 

contracts - 187 

cost  of  pavements,  gross _ -  184 

economics,  relative 18 1 

interests  affected 177 

problem  involved  in 177 

qualifications 176 

rank  of  pavements,  comparative 185 

specifications 185 

Stone-block  pavements 123 

Belgian-block 125 

blocks,  dimensions  of 127 

blocks,  manner  of  laying 127,  130 

cobble-stone 125 

cushion  coat 129 

foundation 129 

granite-block 126 

joints,  filling  for 130 

materials 124 

granite 124 

limestone 125 

sandstones 125 

trap  rock 125 

ramming 130 

stone  pavement,  steep  grades  on _ L 132 

Street  cleaning 172 

methods 172 

snow,  removal  of 175 

sprinkling 176 

Streets,  drainage  of 119 

catch  basins 120 

gutters 119 

surface 119 

T 
Table 

different  road  materials,  proportionate  rise  of  center  to  width  of  car- 
riageway       37 

effects  of  grades  a  horse  can  draw  on  different  pavements .       9 

grades,  methods  of  designating 32 

gross  loads  for  horse  on  different  pavements 9 

life  of  various  pavements,  terms  of . ............. —  180 


6  INDEX 

Table  (Continued)                                                                                               '  PAGE 

loaded  vehicles  over  inclined  roads,  data  for 11 

paving-brick  manufacture,  average  composition  of  shales  for 134 

rank  of  pavements,  comparative  rank  of 184 

resistance  due  to  gravity  on  different  inclinations _* 5 

resistance  to  traction  on  different  pavements 178 

resistance  to  traction  on  road  surfaces 2 

rise  of  pavement  center  above  gutter  for  different  paving  materials 119 

stones,  specific  gravity,  weight,  resistance  to  crushing,  and  absorptive 

power  of 124 

street  cleaning,  rate  and  cost  of 173 

traction  power  of  horses  at  different  velocities 7 

tractive  power  with  time,  variation  of   8 

traffic  census 112 

wind  pressure  for  various  vehicles _  12 

V 

Vehicles,  resistance  to  movement  of 1 

air,  resistance  of 12 

friction,  axle 11 

power  and  gradients,  tractive 7 

traction,  resistance  to _ 1 

W 

Wood-block  pavements 147 

blocks,  laying 149 

creosoting 147 

qualifications 152 


LD  21-100m-7,'33 


3 C 5550 


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